Get a 40 gb or 80 gb "7200 rpm" and get one with 8mb cache. Those are faster.
Then once you get it plug in any power cord thats near it in the case and plug in the second plug on the IDE cable of the first HD. Make sure the jumper is on slave.
I found these on newegg.com:
WD WESTERN DIGITAL 40GB 7200RPM EIDE HARD DRIVE MODEL # WD400JB - OEM, DRIVE ONLY
Specifications:
Size: 40 Gigabytes
Interface: IDE ULTRA ATA100
Seek time: 8.9 ms
RPM:7200
Cache 8MB
$66
►Top Seller!
WD WESTERN DIGITAL "SPECIAL EDITION" 80GB 7200RPM EIDE HARD DRIVE MODEL # WD800JB - OEM, DRIVE ONLY
Specifications:
Size: 80 Gigabytes
Interface: IDE ULTRA ATA100
Seek time: 8.9ms
RPM:7200
Cache 8MB
$85
They both have 7200 RPMs and 8mb cache. Great for gaming.