From what you have said Id be inclined to go for a 10GB disk or more...

Should allow approx

1.5-2GB for XP & Service Pack 1
3GB for Page file (based on 1GB Ram)
3-4GB for your applications

Infact if you can get away with it avoid partitioning your boot disk and get a second hard disk for your samples and work that way. Sure there are other performance issues with partitioning your boot disk but cant find any info on it right now.