yes building new is a good option... i'll show you which parts to get..
just another question...
do you have a link to the powersupply that you have, the 450watt one? or any details. (brand, amps on 12V rail..)
the less you have to buy, the better parts you'll be able to get..
looks like you can keep your hard drive, opitcals, PSU, case, operating system...
for 300-400 $ (i'm guessing you're from the states) you'll get a pretty decent system.
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius