OK, Don't throw the money out so fast.
Lets see, you have 333Mhz DDR ram already. Well, any GOOD MotherBoard you buy will be able to handle it, right?
I'd do this.
Get an 800FSB chip, but go up to the new LGA775 Socket. and get a motherboard that can handle DDR-1 and DDR-2 Ram (They aren't hard to find).
Use your current Ram on the new system with the new motherboard and processor.
Trust me, it'll not dissapoint you.
Save your money up till you can max out the DDR-2 Specs of your board! You'll be happy you did!
Other upgrades to consider which improve gaming
Upgrade to an SATA Hard Drive (load times are WAY lower.)
video card-video card-video card... enough said... But you may want to invest in the new PCI Express.
Think of it all this way... by not going into the high end ream department quite yet.
but get the new socket / chipset motherboard, it will last longer.
the PCI Express will be top end for a long time too.
Then you can use your current RAM and upgrade that later,
but your biggest boost will come from the new PCIE and 800 FSB processor! (get 1MB of L2 cache as well)
I may go out of your price range... but if you save up for another month, or two, you'll be much happier, and have a much more versital computer which you can upgrade other areas later.
a way to put it:
You can afford the Ferrari... But if you hold off, and play it smart, you can have the Lamborghini