I doubt it's an issue with your video card. The 7800 should do a good job. It's more likely something wrong with your SH4 profile or a mod-associated problem. Are you using any mods? Have you tried a new career? A couple more tid-bits of info and you'll likely be running much happier than you were with the 5500!!!
For now--just let the video card handle settings. Don't try to force AA or anything for the moment as that may actually take more time to straighten out your problem. And I'd recommend nvidia's most recent driver as well
http://www.nvidia.com/object/winxp_185.85_whql.html. These are XP drivers--Vista is different but same home page.
Hang in there!!!