CrAsHnBuRnXp has a very good advice on hard drives, nowadays HDD's can get your system a real bottleneck.
So make 2 drives in RAID 0 (RAID 0 is almost 2x of single drive speed) for system, documents and applications. In RAID 0 you don't loose any space, so with 2x80GB you get 160GB total and 2x speed
Anyway, you don't need too much space there so you can get any size from 80 to 250GB. Just remember - drives in RAID should be identical.
Take another 2 drives in RAID 0 for games, you can also store your videos there as they don't run at the same time as your games do, besides you'll have plenty of space in this drive anyway
Get a huge single disk (500GB) and run backups of your important data to it. Important data is documents, application settings, any valuable media that you can't get back from CD's or DVD's.
This configuration is a luxury one indeed
Oh and you'll need to learn how to use JKDefrag - it's your friend for keeping disks clean and speedy
Windows Backup utility will do all that it needs to put your important data on your backup disk. Even better - you could make your old PC as a backup server through the network, but that is probably too much trouble.
Another, not so luxury setup would be to get 1 big disk for OS and store there all important data. Then get 2 moderate size disks in RAID 0 for your games and backups, just don't run your backup utility at the time you play games and use JKDefrag to move your backup files to the end of the physical disk (files closer to the center of the disk are accessed faster).