Id say wait out for the steam summer sale and pick up the valve complete pack. Its all of valves games and last year I picked it up for $25. Its of hell of a deal and then pick up Gmod during the summer sale for dirt cheap too.
Definitely check out Fallout New Vegas. The Bioshock games keep getting put into bundles for like $30 and I really like those. I am also waiting for the summer sale before I buy any games.
Portal 1 & 2 for just over a fiver is a very good deal. If you've never played them I'd highly recommend them. Skyrim is a no-brainer. You have to own this game else you may as well, just like, stop living. I recently picked up CS:GO too, which is pretty fun, but online you'll get your arse handed to you if you've never played it before. Dark Souls: Prepare to Die is on sale right now for about £6.30, but uses shitty Games for Windows Live which is a bit annoying. Fantastic game though.