Get the 8gb. Most apps are less than 5mb. I have 2 pages of apps including monkey ball, hold em, and crash and it takes up 497mb so you'll be good with the 8gb.
Most apps are pretty small (often 1MB), but some of the more graphical ones are close to 100MB (e.g. Spore and Texas Hold'em). If you only have 1.5 gigs of music and you're not intending to uploading much video, then 8gb should be fine.