Downgrading from GF 6600GT to Radeon 9800 Pro

Due to issues with shimmering textures in FS9, I've decided to get a R9800 Pro. I'd like to ask R9800 users what is the performance hit I can expect in FS9? I've never used a Radeon card before, so is there anything I need to know w.r.t. FS before switching? Performance elsewhere doesn't matter as I'm no gamer. R9800 is the only card within my budget so other options are out of question. I frequently use addons such as LDS767, PMDG747/737, Simflyers/Fly Tampa scenery with FSGenesis, ASV and FE. SYSTEM: AMD ATHLON64 3000+, 1024Mb 400Mhz DDR RAM, ASUS K8N Mobo.Any help will be greatly appreciated,Thanks,Keith.