Difficult to say for sure whether you will get decent framerates, it will vary from game to game and vary within each game. You should bear in mind that you are at the low end of minimum specs and that usually means the game is barely playable imo.
Are you sure you have a 4890? If you do have a 4890 this is a better card than a 4850. You need to post back if it's not a 4890 and tell us what card you have at the moment and what PSU you have (4850's suck some juice so you need a good PSU to deal with that).
The higher the native resolution i.e. 1440x990<1650x1080<1900x1080 the less frames you get with a given graphics card as a rule. A 4890 could handle 1900x1080 adequately, but you really need to upgrade your CPU (and probably mobo and PSU too).
I have a 550 watt Corsair. I havent got a gpu yet. im wondering whether to get the 4890 or 4850. I wont be able to upgrade the mobo and cpu for a while thats why im wondering which card would perfrom better with this bottleneck.
Ah I'm with you. The 4890 is the better card for sure but it will be severely bottlenecked. I would suggest you get a 4850 and a better CPU (cheap core duo) for the money you would spend on a 4890.
1. 4890 is better card and can handle 1900x1080 res but not with your CPU.
2. 4850 struggles at that res but fits your system better
3. Your CPU is the weak link in your comp by a country mile.
4. your PSU is fine and adequate.
Check AMD bundles, you can get a decent cpu and a motherboard for good money. Someone will probably find one for you if you post in the cpu and components forum and say what your budget is, I dont have time now sorry.