A fast scratch disk makes a big difference in some apps. I'm assuming this is for photoshop, right? I'd personally go with an SSD. A velociraptor is not available in 80GB (the smallest one is 150gb), and capacity isn't that critical as a scratch disk. I'd lean towards either an Intel 80GB X25-M or a Kingston SSDnow V series 40GB. The Intel is faster, and the Kingston is an amazing value. The Intel is $215 on newegg right now, which is a great deal.
I wouldn't worry too much about lifetime either - the lifetime of any decent SSD should be quite a bit longer than the computer it's in.