As you're posting this on the Doom 3 forum I expect you'll be using your card a lot on Doom 3. In which case, the x300 might not be the best way to go. The Geforce 6200 tramples all over it in Doom 3 benchmarks; the x300 manages just under 20 frames per second in low quality at 1024 x 768 resolution, whilst the 6200 manages a much better 42 frames per second. Indeed, the x300 only reaches playable speeds in
very low quality, whilst at this setting the 6200 runs away into the distance. Although the x300 does make up some ground in other areas, almost on a par with the 6200 in Half-Life, it also falls flat on FarCry benchmarks too.
Check out this comparison for more information.
I don't know where you live or how much you're getting it for, but according to Dabs.com UK, you can get a Saphire x300 for £40 and a 6200 for just over £55. Not a
great deal more, especially if you can get money knocked off from your friend.
But even if the 6200 is too much for you, you might want to look into how much some of the other cards featured in those benchmarks in the link above cost.
Of course, these are all low budget cards; you'd get a lot more performance out of a more expensive card, but if you don't have the cash, you don't have the cash, right?