a x1950 pro will play those games well, but a 3850 is about twice as powerful.
for those games i think 256 MB will do.
you are using a 64 bit OS arnt you, otherwise you will not see all 4GB of ram, as a 32 bit OS will only see 4GB of memory in total. this includes graphics card memory and cpu cache. you will most likely only detect about 3.7 GB with your setup