Yes, most PCs can run those games...I'd say about half of them are older and can run on nearly any PC. Source engine games are much better looking, and require slightly newer PCs...but if your PC was built within the past half decade, it should be fine.
Valve's games scale down to various PCs very well. There are system requirements available on the Valve Store somewhere I'm sure.
Space...well currently Steam takes up 30 GB on my system...but I have quite a few non-Valve games like Company of Heroes, Call of Duty 2, as well as a TON of free mods that utilize the Source engine. I'm guessing all these games will be under 20 GB.
Steam is 100% free to use, it's only the games that cost money to buy. You buy the games, and then you can play online with Steam's servers, download and play your games on any PC with internet access, use its Community features, etc for free forever.
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )
