With the prices on drives having come down over the last few years you can grab a good sized drive to allow room for additional softwares and still not pay a high price. A WD 40gb ATA100 ide drive goes for $38.99 at
Newegg.com - Western Digital Caviar WD400BB 40GB 7200 RPM 2MB Cache IDE Ultra ATA100 Hard Drive - OEM and that's a small drive with 80gb models seen for a few only a few dollars more.