GTA IV was awesome for the computer, runs like butter for me and I have a pretty dated machine by most people's standards
Either you are one of the few people that knows the capability of their pc and sets up the options appropriately or your definition of butter is different than mine. XD
I guess that could be a bit misleading, my monitors native resolution is 1366x768 so I can push the settings a bit. Never had any stuttering though, and never felt like I had to limit myself in the explosions department either
I already got IV as retail for PC. I'm waiting for the Episodes to get lower. I'm pretty sure they'll eventually go on flash or daily deals since they're very popular. Even if they don't, $9.99 is a very fine deal as opposed to $29.99 retail.