I would save up more money and just buy 1 piece at a time until you build a new system... Upgrading your current system seems pointless as you are probably running really old hardware like AGP and IDE. Just makes it hard to upgrade efficiently.
Some parts to ponder:
$220 - Core 2 Duo CPU (mid range core2duo, but not by any means slow)
$100 - ATi x1600Pro PCI-e (have this in my laptop, plays everything just fine.)
$50 - ECS Core 2 Duo Motherboard
$100 – DDR2 Memory
$60 – ~400w Power supply
~$530 (Maybe > $500 during / after Christmas..)
Optional
$30 - DL DVD+/-RW
$70 – 250GB 7200rpm SATA HD
$50 – Nice new black Antec case
(All from newegg)
The system above would smoke anything you can put in your current system, and is almost top of the line right now. Just ask Steve, he got a nice new shiny Core 2 Duo
. Plus it will be easier to upgrade because all the hardware and slots (PCI-e, SATA etc) are current (for now…).