Welcome to Xbox land. Many good games available but depends how far you wanna go back. You've got a good varied collection there but part from Forza, they are fairly dated but still worth playing to see what you've missed out on. Bear in mind when playing say for example Mass Effect, this was released a couple of years ago so may seem dated to you but if you're not a graphics fiend than you won't mind. Plus it will help set you up for the up coming sequel which will feature an up to date tech engine.
When it comes to racing games, well most of them available on 360 are also out for the PS3 so you may have already played them. I know you say you like sims so Forza is the best for that. Though it's not a sim, PGR 3 or 4 are a steal right now and exclusive to the 360. Gameplay wise they are very very enjoyable but are fairly arcadey.
