I would go for an SSD since its much faster. If you can buy Intel X25-M 80GB SSD, you would have a formidable systems performance. You can still use large mechanical drives for mass-storage, but your applications, operating system, games and other active documents would be on the SSD.
If you have an ultra-fast i7 system but all those mighty cores are waiting on your HDD, you won't see the huge performance potential of your system. Try to invest in an SSD. If 80GB is too few space, try to put two of them in RAID0 to get one drive of 160GB. It should be quite safe to run SSDs in RAID0 since it can't fail like mechanical disks can.
If you can't go for the SSD option, two velociraptors in RAID0 would be a good alternative. But make sure you either use Windows 7 or Linux/BSD. If you use older windows versions, lots of potential will go to waste by a misalignment caused by ill partitioning. You can manually correct this misalignment. Google for it.