First thing you need to find out is if your motherboard has an AGP or PCI-E interface for graphics cards.
As far as cards go, I would recommend at minimum a GeForce 6 series card, but those are old and limited compared to more recent cards that to a lot better, so a GeForce 7 series card or even 8 series card, but I don't know if there are any AGP versions of GeForce 8 series. ATi/AMD does offer pretty recent cards with AGP interface though. Also, as far as I know, if you go for GeForce get at least 7600 GT (which is a good card), but don't bother with 8600 models. 8800 GT on the other hand is awesome as far as price/performance ratio is concerned.
Do not get an ATi Radeon 1000 series card (1500, 1600, 1800), they tend to have problems with the normal maps.
A GeForce 7800 or Radeon HD2000 series card might be good choice depending on your other system specs. Getting a latest GPU with all bells and whistles has little point if your CPU or memory form a bottleneck.
As far as memory goes, FS2_Open and MediaVP's benefit most greatly from memory, so you should go for 512 MB or at bare minimum 256 MB to use the MediaVP's with good consistent frame rates.
