Well,WoW benefits from faster CPUs so if you only upgrade your VGA,your current CPU will cause major bottlenecks preventing your VGA to run at its full potential so the best thing is to upgrade both VGA and CPU
Cataclysm doesn't benefit from more than 3 cores but it is dependent on clock speed. there will be no difference between triple or quad cores in Cataclysm. This came from the Cataclysm performance review here on Tom's Hardware, check it out if you haven't already.
I run a ASUS TOP gts 450 and a athlon II x 4 3.0 Ghz and play WoW at 75 fps in ultra so i can tell you that'd be a good option as long as your mobo will accept the am3 cpu .. after a little research it looks like you can use the athlon II x4 but you will need the latest bios update
can only say what i'm getting on my pc, i can sceen shot and image shack link at home showing the processor and gpu as well as in game showing the fps being recieved. not i get 75 fps at a lower resolution than you 1280x1024.
however i still get 60 fps on my 37" lcd tv at 1080p i figure its limited at 60 fps on that one due to 60 Hz being the maximum refresh rate
I liked the toms review alot, don't get me wrong, and it was done in a very logical way to put stress on your system. the draw distance and sheer amount of graphics to rendor in a flight is much higher then you will experience on the ground while questing, doign dailies, or instancing.
when in cities my fps will drop into the 40-50's on the 1080p tv btu stays 75 fps on my monitor in ultra. also while flying i will get some m,inor fps dropping in flight but i'd still classify it as smooth (i consider anythign over 30 fps smooth and 20 -30fps playable) even in battle grounds with full effectsa and clashing teams i i still never even drop below 30 fps when i check.
i think the results in the tom's review had a purpose to show when its truely stressing your system however WoW is not going to be constantly straining your system that much
For cataclysm a triple core or quad core Athlon or Phenom II will perform the same, but clock speed makes the biggest difference. What quad are you considering? If you are debating between an X4 at 2.6 and an X3 at 3.2 the X3 wins hands down. I've been doing a lot of research on this because I'm building a computer for a co worker specifically to play Cataclysm. The GTS 450 is perfect for it.