Well, seeing that you only want to use this computer for WoW then I would upgrade you CPU.
The video card actually rips through WoW as I had a 7600GT on a AMD Windsor X2 5600+ overclocked to 3Ghz. My Ram is and was at 860Mhz.
In that system the computer ram WoW at 1024X1280 maxed to 60FPS. If you wanted to keep the cost under $250, I would that could be done.
What you need for an upgrade of you Processor -
A New Mother Board - $110
New Ram - $32
New CPU - $60
The CPU - Link
That CPU is a Black Edition so it comes with an unlocked multiplier. With that you can take this CPU to around 3.0Ghz and over that from some reviews I've read. It's better than the CPU you have by the speed of the Hypertransport, Core Speed itself, and has added L3 Cache.
It's Dual core but it should last you a while if your only playing WoW for now, but this CPU is also great with some harder core games.
The Motherboard - Link
That has a SB750 South bridge so it should help you take the processor above to higher clock speeds when Overclocking. Should get it around the 3.2Ghz's I 'think'.
It has two PCI-E slots so make sure you 7600GT is a PCI-E card and not a AGP card.
The Ram - Link
This is what I think held back your system before big time. DDR Ram has a max speed of 400Mhz effective, but remember that's doubled. So the fastest Ram you could have is set at 200Mhz (Double Data Rate). With this new motherboard and this Ram, which is running at 1066Mhz effective speeds(Or 533Mhz actual speed), your computer will be much more responsive.
And of course, you don't have to Overclock your CPU, but you would be missing out on some nice speeds if you didn't.