Look at click here it has a lot of stuff about graphics cards including benchmarks comparing the different cards. Regarding whether you should upgrade it depends on the games you're actually playing online. If its an older game then there is probably very little point as you wouldn't see much difference. However if its newer games ie out in the past year or so you probably would see a significant difference but thats only on the latest games.
As a test look at the games you play at the moment and look at the graphics options for each one. If you can use all the facilities of the game and it still runs without slowups and stuttering then your card you have now is fine. If however theres a lot of settings you can increase and it causes the game to crash, slow down or whatever the you're really not seeing the game at its best and perhaps its time to think of an upgrade.