it also depends on what you do with your computer to what hdd you get.
If you use your computer alot of gaming, or programs that acces your hard drive alot, a SATA drive (if your motherboard supports it) might be the way to go.
If your motherboard dosn't support SATA another 80Gb should do you fine, as your current hdd is reasonably new - it shouldnt die any time soon. If your a serious gamer, you might consider a 120Gb as games are beginning to take up more and more space (Eg UT2004's 5 disc install).
I've got a WD 120Gb SATA - partitioned into 3 partitions, and I noticed the difference when I upgraded from my previous 40gb PATA drive. Ive got 2 40gb drives, one for all my music and other media, and the 2nd for backup, which holds my ghost image and important files.
Price wise, SATA drives have come down quite alot in the past 6 months, I payed $170ish for my 120Gb drive, you can get them now for arround $145 if you look arround hard enough. An 80Gb PATA drive will set you back arround $100 for either WD or Seagate.