edit: no need for a PSU now
assuming you have a <500W PSU
first off id recommend getting something like this
http://ncix.com/products/index.php?s...c&promoid=1280
its an antec 650w PSU for $65
then u can get a bundle from ncix
like this one
http://ncix.com/products/?sku=59760&...Bundle%20Deals
for $360 (after rebate)
thats already $425, and then you can purchase 4gb or 8gb of ram depending on how much you want to spend, 4gb would range you around $50
mind you, I'd recommend not always going for bundles to save the couple bucks (unless you were like me, building a budget system so every couple bucks makes a difference) and you can choose a better motherboard.
But if you were to upgrade, I'd highly recomend the new Intel Line of Sandy Bridge, because off the line w/o overclocking they perform great
if interested, get an aftermarket heatsink, do some cable management in your current case so improve airflow, then you can overclock without really needing to purchase another "upgrade"