I've had an Adaptec 2940, 2940U2W, and 29160, which have all been excellent cards, and I would highly recommend any of them. (Particularly the U2W or 160). I'm in the process of upgrading to an Adaptec RAID 3200S, since I haven't plugged it in yet, I don't know how well it performs, but I'm optimistic because of my past experiences. In the 9-18 GB range, The Quantum Atlas 10k II and the Seagate Cheetah X15 would be the fastest available hard drives, for a cheap drive, you'd be looking at something with a slower spindle speed like the IBM Ultrastar 9ES or 18ES. You can also find pretty good deals on Ultrastar 18LZX (10k) and Cheetah 18 drives. You should probably get a 10k drive for booting (9-18 gig) and if you need more space, but don't need the extra performance, go with a 7200 rpm drive for data (they go up to 180 gigs right now, although that drive is hardly cheap).
The upgrade to SCSI is definately worth it, if you can afford it. I'm hesitant to try burning a CD while I play Quake, or a similar game, but I've done other things, such as encode mpeg files, and other similar CPU intensive activities while burning CDs, and only rarely get bad CDs (99% of the time that is due to configuration issues)