So don't play it anymore?
PC gaming is quite nice. You know you want to drink the Kool-Aid.
That was the most expensive decision I've ever made... Long story short, I started off playing PC games via Boot Camp on my Mac, got hooked, built a PC and have since built a sim racing rig which will soon have three monitors on it. Not cheap, not by a long shot! Then again the games are far cheaper than they are on PS3, new games are quite often £10-20 cheaper, and ludicrous sales and pay-what-you-want indie bundles do help ease the pain of spending £470 on a top-of-the-range graphics card (like I'm going to soon).
I might have imagined it but I thought Starhawk was already out for Plus members, but I imagine it's out soon. I don't actually know, but I think that was obvious.
Assassin's Creed is indeed worth a try if you've not touched it before, but I'd start with AC2 if you don't care about the story because the first was more of a tech demo, there wasn't much to do. I mean I personally enjoyed it, but it was quite heavily slated for being too repetitive.
GTA IV, yep, excellent game too, but unless you can find it for very cheap it might be better to find some other games for a few months and get GTA 5 later this year (I think?).
Another I've just thought of is Just Cause 2, it's not perfect but you try and name another game where you can do even half the stuff there is to do in JC2. It's got a properly massive open world, so big that you probably won't see about three quarters of it if you only do the main missions (which you can't actually do anyway because the game makes you cause trouble to progress) and slightly RPG-style weapon and vehicle upgrades, a ludicrous plot and the cheesiest voice acting I've heard in a long time, but you're supposed to play it like it's an Arnie film, no, worse, a Van Damme film.