Video Card Requirements for PC gaming?

Hi, i was thinking about getting a new video card in hopes that when Mass Effect 2 comes out my PC can run it smoothly (not necessarily on full settings, medium would do fine :D). I was looking at purchasing something for about $100-120 (Australian currency that is). I really don't know anything about hardware unfortunately and i'd really appreciate it if somebody could help me out :).



So here are my specs (hope these are the right ones);



OptiPlex 755

Intel Core 2 Duo - 3 GHz

2 GB RAM

ATI Radeon HD 2400 (256 MB)



So anyway, the first question is:



1) At the above price (100-120 AUD) what sort of video card would you recommend, and just roughly would it be able to run Mass Effect 2 and most newer games smoothly on medium settings. I was looking at a ATI Radeon HD 4670, but tell me if you think that's not up to standards enough.



2) Also possibly more importantly, how would i know that it would be compatible with my PC. Will i need to open it up?



Thanks a lot in advance :).|||by the looks of things you been doing your homework, yes if you were to get the ati 4670 i could play mass effect 2 on med because i have its older brother the hd 4650 and can play crysis on med without lagg on 1600x1020 res and the rest of the components looks fine core 2 duo 3.0Ghz i have the 2.55Ghz one, 2 gb of ram might want to buy more, and thee old HD 2400..... i say your good from there