Alrighty, my reccomendations are:
Morrowind III: Elderscrolls- Uber Open ended RPG, has expansions to buy and user created ones as well.
Farcry-Very nice looking and playing FPS, has smart A.I. and is a very well made shooter, though its insanely hard.
Unreal Tournament 2004-Great multiplayer FPS, has vechicals, super supported mod community, and is great for mindless killing.
Battlefield 1942 or 2-Both are amazing games, I think they are equally good in their own way, so if you're short on cash, you can pick up 1942 with expansions for cheap and then instal the "Desert Combat Final" mod for modern combat. If you got the money, 2 is also a great game with better graphics and a higher ability for informing players on what to do. MULTIPLAYER ONLY! Single player A.I. is retarded on both games.
Guild Wars-Great MMORPG, it gets old after about 2 weeks if you don't like fighting other players, but thats more time than most games can offer anyways so I highly recommend it. If you do like Player vs. Player it can last you for a very long time. Expansions will be released in the future if you want them, but you don't need them.
World of Warcraft-Supposedly Great MMORPG, This is one of those, omg game never really ends RPG's, it comes with a monthly fee, but judging by the shear number of players the game has, it doesn't seem to deter too many. I admittedly don't like that style of game, but its a "true" MMORPG where players can randomly run into each other out in the wilds and has crafting, mining, fishing, cooking, etc. all that boring everyday stuff that doesn't belong in games

. Its supposedly the best MMORPG out right now, so if you've got a steady pay-check and want the best, go for it.
Microsoft Flight Simulator 2004-Awesome Flight Sim, though I'm guessing not many people like these anymore (since I've seen, what, 2 flight sims come out recently), flight sims are still my favorite genere. You can't get too much more realistic than M$' flight sim and it allows you to fly over real world locations which is really cool. If you're into flight sims, pick that one up.
Neverwinter Night Platinum-Excellent 1 or Multiplayer RPG, the graphics are a little dated now days, but for streamlined D and D RPG action, you can't go wrong with Neverwinter Nights. The platinum comes with the two expansions and is only $30 at BB (25 GGC).
Heros of Might and Magic 4- Great strategy game, build up armies of troops from a castle and give them to your hero so he/she can capture more castles to train more troops until you control an empire. Not everyone's cup of tea.
and finally...
Half-Life 2-IMO best one-player FPS action you can have on the PC. If you've got a computer able to run it, buy HL-2. Great story, great action, amazing graphics. It's still pricey, but I'm sure here on CAG you can find it for under 40$.
Those are my suggestions. However, those are the games that you could buy. You must not forget the plethora of free games available for download. I will quickly go over the best I've found so you can download them when your short on cash or need a new game.
Gunz: The Duel- Sweet action 3rd person shooter with melee weapons in addition to guns. Has an RPG quality in that, you kill people and get exp and money. Exp levels you up to use new items and money lets you buy those items. Very nice for a free game, though its still a little buggy since its in beta, but it will remain free even when it goes commercial.
Download at
www.gunzonline.com
Conquer Online- Diablo IIish MMORPG with nice handdrawn backdrops and 3D characters. This is basically a generic RPG complete with level grinding, loot finding, and Player Vs. Player action. You can even mine omg! Seriously though, its quite fun, but the PK system lets you kill anyone anywhere, so you gotta be careful with your stuff since you drop it when you die.
Download at
www.conqueronline.com
Gunbound World Champion- The classic game of artillary, only with crazy things instead of tanks. Use a variety of weapons to kill people in this turn based strategy shooter in the vein of artillary. Apparently this game was better before its "World Champion" edition, but it seems just fine to me though I never played it before. Killing people earns you money to pimp-out your little dude with stuff that improves your stats. Entertaining and Fun time killer.
Download at
www.gunbound.net
Maple Story-Another free MMORPG, only this one is 2D side-scrolling. Its also all in anime style so that may or may not appeal to you. Typical pick your class, kill monsters, level up action, but the side scrolling is a nice break from typical RPG's.
Download at
www.mapleglobal.com
Thats the best free games I've found. I hope this "guide" helps you, and happy PC gaming!