Yeah any Bloomfield CPU will fit, 920's are usually the cheapest and often overclock to the same speeds as their more expensive counterparts.
I'm still rocking an 8800GTX (stock & under water) and yes while I'd like to upgrade it, it still enables me to play at decent frame rates. Valve games such as Portal 2 & TF2 still run nicely with 4xAA &16xAF @ 1680 x 1050), BFBC2 plays pretty well as does BF3 if your willing to drop to low/medium and lower the eye candy. 2 in SLI as a budget system should however cope much better. You might find SLI support a bit patchy for newer titles as generally they'll be optimised towards newer hardware. These cards were monsters back in the day and are certainly not low end, just a bit old.
The key phrase being "Were", yes they were good cards, but compare them to cards nowadays and they will be classed as low end.
They are still more powerful than today's low end cards, 2 together would be on par with a 460 768MB in terms of grunt (even if the don't have Tessellation or handle Ambient Occlusion as well as modern cards) which can still be considered mid range and are more than adequate to play most of today's games at full detail. You may need to turn down an option or 2 in a couple of the more demanding titles. Overall a modern card is more desirable for the reasons listed above but in a bind a SLI 8800GTX setup would do quite nicely. If you use Vsync then the actual power draw won't be that much more when playing games. I found when upgrading from my old 9800GTX+ SLI setup the difference was only 20-40W or so depending on the game. Idle power can be reduced to with Goodbyte's NVGPUpro
I would sell 'em and get a 460 or a 560Ti 448 edition if you can stretch it. You don't want to scrimp on a GPU in a gaming rig.
The 8800s are not worth the effort - having to SLI them just to get playable rates should be enough to discourage you. As suggested, sell them and put the money towards something useful. A £500 budget is not to be sniffed at, so build yourself a decent little rig and don't hamstring yourself with ancient tech.
8800GTX is roughly equivalent to a GT545, so these are definitely now low-end parts. Also, do NV even still write SLI profiles for cards this old?
I got the parts for a friend who had a £400-500 budget a couple of weeks ago, here is what I got. i3 3220 + Gigabyte GA-H61MA-D2V + 4GB Corsair DDR3 - 1333MHz £168 You can add an addional 4gb RAM for £7 and upgrade to an i5 3450 for £40. 2GB EVGA GTX660 superclocked £178 I won't link the HDD I got because I ordered a 2.5" instead of a 3.5" but thats around £40-50 for 500gb or £55 for a 1TB seagate 550W be quiet psu £44 case ~£20 DVD drive ~£12 That's £477 for the cheapest motherboard/CPU/RAM combo or £524 if you want the extra ram and i5 CPU. Just remembered that he didn't need a OS though so that will take you quite a bit over budget
i think i may just get the rest of the rig together with these in see how they cope and then make a call. cheers Deders though for the info on the cpu. even 930's or 50's seem to be going quite cheap on Ebay.