Here's a link to your mobo's manual:
I agree with BiGyLiLuPdAmIdL in that your graphics card is slowing you down, and your system is starving for RAM. According to your mobo manual, using PC2700 works with 2 DIMM slots with a max of 2gig, PC3200 with 1 DIMM with a max of 1 gig.
You could go with PC3200, max of 1 gig, to increase your speeds.
Your mobo can handle up to a 800 FSB, so if you wanted to upgrade your CPU you could look for the fastest socket 478 you could afford.
Celerons work, but they can't hold a candle to a full P4.
Also, make sure your system is clean from viruses and adware/spyware. Turn off any unneeded programs running in the background (keep your anti-virus and firewall running). Make sure your hard drive is defrag so your comp can access its info quicker.
If you can squeeze out $285, then look at these:
BFG Tech OC Geforce 6800GS 256MB AGP 4X/8X
Lifetime warranty, Core clock 370MHz
$194.99 + $5.99 Three Day Shipping
Patriot Signature Series 1GB DDR 400 (PC 3200)
$78.50 + $4.81 Three Day Shipping