You may be able to quantify your options a bit, assuming you have a mac already.
Set your dock to not auto-hide and put Activity Monitor (from Applications -> Utilities)in it. Right click on the icon in the dock and choose Dock Icon / Show Disk Activity. Now open the programs you think you'd use and do what you expect you would do in them, keeping an eye on that icon. Now as I understand it OS X keeps closed programs in memory for reuse if you open them up again until it fills up the existing ram, then it starts recycling (witness the dot under your Safari / iTunes after you close them) so you could expect that after the initial program load your 8 Gigs of ram will work to minimize the work your drive has to do.
On a somewhat related note, it's not uncommon to kit out a performance / gaming rig with a small HDD or even SSD for the OS then add in a large capacity (slow) internal or external drive for your music, movies and other programs where speed doesn't matter much but having them available does.