Do you just need to update you GFXCard? if so than I would suggest that, and get it for PC. If you need to do a full PC makeover it may cost more than a PS3 and you should just get it or console.
Xbox seems to be the best option, as it was yet again tailored to that community, and will have much more players, and I bet your friends will play there too. PS3 Community seems too small, not really worth the price you would throw down for a PS3+Games.
But if you can afford to get a PC and would rather do that, I would personally go for PC.