What graphics card(s) meet my requirements for a game?

From the NVIDIA site, looks like any GeForce 6 family of cards would meet your requirements. In reality, pick one that ends with a x600 or higher (i.e. the 6600 GT, 7600 GT, 8600GT, etc...). The x600 level cards are what is generally accepted as entry level gaming cards for more serious gamers. If you are playing a less graphically intense game, you may be able to get away with a x400 level card. If you plan to do serious gaming though, go for the x800 level cards (just be ready to pay a lot more money)

I have the MSI NVIDIA 8600 GT OCX (512 MB) card and it works decent for games like World of Warcraft on higher settings. I do get some frame rate issues, but they are tolerable. I'm running with a 250W stock PSU so the power requirements are barely met, but I haven't notice any problems related to a lower PSU.

As for purchasing a card, take a look at NewEgg.com. While they may not have the absolute lowest prices, they are fairly priced and you shouldn't have any problems with the purchase from them. I also go through Pricewatch.com to find other vendors, but I always make sure to check customer reviews since there are some vendors that aren't always responsive to problems or ship wrong parts.

** Edit **

Forgot to post a link so here's an update. You also didn't mention which video adapter you have. The one below is a PCI Express x16 but if you have an older AGP port, please update your question and there are options for older cards.

In addition to the port, please clarify that this is a desktop system. If you are doing this on a laptop, you are pretty much out of luck since most don't have an upgrade option for video cards.

Yes, the mobile Intel 4 series perform better than a 256MB ATI. Plus the game civilization 5 isn't that graphic intensive, infact you only really need to worry about a higher grade graphics card when theres a lot of graphics to process QUICKLY (games like Call of Duty)

PCI

AGP

PCI Express

Nvidia 8400 GS

ATI HD3650, ATI HD 3450, HD 4350

300 Watt or greater power supply is recommended.

http://www.newegg.com/Store/Category.aspx?Category...