Help Finish This Barebone Kit Please

Hello everyone! Lately I purchased a barebone kit in which the weakest part of it was the graphics card and the power supply. I'm a WoW player and I raid a lot, and lately I have been running across big fps issues causing me to drop all my settings to low and still barely holding 16fps. I bought this barebone so i could run games smoother, and now i am going to put the finish touch on it by getting a psu and gpu like mentioned earlier and i need your help !

Here are my specs

Amd Phenom 9500 Quad-Core
ECS MCP61PM-GM AM2 mATX motherboard (Nvidia chipset)
Bestec 300w power supply
ati radeon 2400xt
3gigs of ram
500g Seagate HD
Windows Vista 32 bit Premium

Now Im at a standstill with what graphics card i should get, because A) this is a AMD system, in which they usually run ati cards and B) the mobo's chipset is Nvidia.... so maybe i should get a nvidia card ???

I have figured out that I will be purchasing either a 500W Corsair psu or a 650w Corsair.
Question is.... 4850 or 9800gtx+ to suite this system and motherboard especially.
I want to be able to run Wow maxed including shaders on my 17" Sharp.

Thanks all