There are two versions of the GeForce GT 545; a GDDR5 version and a DDR3 version.
While the GDDR5 version is more powerful (faster shaders, core and memory) it has a TDP of 105W. The DDR3 version has a TDP of only 70W. So in your circumstances the 70W version might be the safer choice.

It's an unfortunate fact that these CC2.1 cards can only use 2/3rds of their shaders here, so either would work as if they had 96shaders rather than 144. Only the high end, power hungry cards are CC2.0, and none of those would work on a 300W PSU.

Also worth noting is that the Kepler (600) series cards are due to start turning up sometime in the next 3months, or so, if you wanted to wait and see. These will be based on 28nm technology so they should be more power efficient &/or perform better. Exactly what turns up, when, how they perform, and if a new app is required is of course unknown. So it could be this month or in 6months time that people are using Kepler GPU's to crunch with.
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At SETI.Germany we've got some people crunching on i5 2500's using the integrated GPU and nothing else. They manage to draw ~100 W from the wall with a little undervolting of the CPU. So your system should draw ~110 - 130 W without the GPU. Factoring in 87% power efficiency of the PSU (a modern 80+ Bronze or Silver) would mean the PSU has to supply 96 - 113 W in your case. There's plenty of room for a 70 W GPU. Personally I wouldn't worry about putting a 110 W GPU in there, as that would put the PSU at just 2/3 load. 150+ W GPU is out of the question, though.

I'd definitely go with the GT545, as the difference to the GT520 is too large. And I doubt the actual difference in power draw between the GDDR5 and the DDR3 versions is as large as the TDP makes it seem. NVidia probably plays it safe with the high rating for the GDDR5 version.

MrS
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Cards I tested while running GPUGrid tasks use well under the full TDP. Around 70% is about the norm, but this changes with app and WU.
My main concern is an OEM PSU (tend to be poor) and the fact that a PCIE 2 motherboard only supplies 75W per PCIE slot. If you use a 105W card you will need a 6pin power cable from the PSU to the GPU (or a dual molex to 6pin adapter).
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It's an HP machine. I'd expect the PSU to be cheap, but not to be completely rubbish.

MrS
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GTX460 is good value, but borderline with the OPs current PSU. Interesting question: could you swap the PSU for a beefier model? We've got some Dells at work, which use custom designs.

If another PSU is an option, I wouldn't go for the 120 - 150$ Corsair at 600 - 700 W. That's far too much power for a stock i5 and a single GPU. 400 W would do, 500 W would be plenty.

Personally I'm running an i7 @ 4 GHz and an overclocked HD6950 with unlocked shaders.. with an Enermax Modu 87+ 500 W. It's 80+ Gold (good for a 24/7 cruncher which draws a bit shy of 300 W from the wall) and all you notice from it is a slight breeze of cold air out of its back. And even this would be overkill for the OPs system.
I'd suggest the FSP Aurum 400 W.

MrS
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OP stated a preference to not upgrade the PSU. Also, OEM systems tend to use non-standard connectors (power to board, on/off switch, reset...). So unless there is a change of mind and the OP can confirm standard connectors to the board, it's a non-starter.

If he pushes the PSU by using a GTX460 (TDP 150/160) it's almost inevitable that the PSU would deteriorate faster, and he would be facing problems. A GTX560 would probably use slightly less power than the 400series counterpart, but I think it would still be too power hungry. A GTX 550 Ti has a TDP of 116W. Can't see it using more than 85W, and might draw under 80W. Riskier than the 545 but perhaps worth considering? Would still need to make sure the PSU has two free Molex connectors, be prepared to fork out the extra, and take the risk.
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Nah, that list is totally ridiculous. It's erroring on the safe side, assuming a beefy system around the CPU and, most importantly, assuming a crap PSU.

GTX460OC: quality 400 W is fine, 500 W plenty (unless you've got 2 OC'ed CPUs). GTX590 is a different beast, though, and quality 500 W will be borderline. 600 W should be OK.

Modern PSUs (and you really shouldn't use anything else for a cruncher, otherwise the higher electricity cost due to the low efficiency will quickly exceed the cost of a new PSU) provide practically all their jiuce on 12 V (~90% or so).

MrS
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It does not say "on a single rail". This is probably factoring in 130 W from the CPU, completely at 12 V, as well as 12 V over the PCIe slot and the 2 GPU connectors. That's 4 different sources of high currents, so a PSU with 4 rails and a maximum of 38 A combined from all of them could do it, provided it's balancing the load well.

The GTX460 has a TDP of 160 W and the actual card draws about 115 W in games. That's just 13.3 A and 9.6 A at 12 V!

Let's error on the safe side and assume 150 W for GPU-Grid, plus a CPU drawing 130 W, together with a 90% efficient mainboard yields 150 + 130/0.9 = 295 W, or 24.5 A at 12 V. That means with a 38 A PSU we'd be at 65% load - easily within the comfort zone of a modern and efficient device.

MrS
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