Oldtimer I'm using an ex-Mr. Zooly Nvidia gt460 with 756mb of vram. By todays standards this is now getting past it however I think it's fab! I can run Rise of Flight with pretty high graphics in the 30-50 fps range. That don't sound too huge but the game itself runs so smoothly that there are no problems whatsoever. It can also run CLoD in the same range of fps with high aircraft and cockpit detail and low land detail. However, the game is........ lumpy. A combination of the limitations of my system and the the coding of the game itself.
I can run IL2 with everything maxed out at a fixed 60fps. Interestingly HFSX 6.0 runs much more smoothly than my UP3 install.
There seem to be quite a few of the 460/470/480 family of cards on Ebay at present. A quick run through found these for example;
http://www.ebay.co.uk/itm/Zotac-GeForce-GTX470-nVidia-1280MB-/251099234788?pt=UK_Computing_Computer_Components_Graphics_Video_TV_Cards_TW&hash=item3a76ae41e4
http://www.ebay.co.uk/itm/Sparkle-Nvidia-480GTX-GeForce-1536-MB-SXX4801536D5NM-Graphics-Card-/261057278117?pt=UK_Computing_Computer_Components_Graphics_Video_TV_Cards_TW&hash=item3cc839f0a5
Or for a few more quid you could punt up for the Nvidia 560 series. I think a few of the guys use variations of these cards with very good results
http://www.ebay.co.uk/itm/NEW-inno3D-GeForce-GTX-560-Ti-with-CUDA-1-GB-DDR5-PCI-Express-2-0-/190697529209?pt=UK_Computing_Computer_Components_Graphics_Video_TV_Cards_TW&hash=item2c66755f79
http://www.ebay.co.uk/itm/Gigabyte-NVIDIA-560Ti-560-Ti-OC-Edition-900MHz-factory-OC-PCI-E-Graphics-Card-/190697576483?pt=UK_Computing_Computer_Components_Graphics_Video_TV_Cards_TW&hash=item2c66761823
Also, if your motherboard supports it I'd get more ram or move up to 4gigs of DDR3. Time to consider this too;
http://www.software4students.co.uk/t/brands/windows-7