Asking others, "what are some good games?" is like asking, "what CD's should I get to listen to".
Like audiophiles, you begin in one genre and accept others over time.
First a disclaimer. Buying PCs suck. A fast one is expensive and a slow one is frustrating. They technologically depreciate faster than cars. Games today are very expensive because they don't run on yesterday's system. Now that the generational change from AGP to PCI-express is in full force, a simple component upgrade is not possible. Buying FEAR for $50, may end up costing 2grand.
Let me offer some games of the past by genre. Most of these games are 3 to 5 years old and should play on anything that that can comfortably run XP with non-integrated video. The other nice thing is that these games are now very cheap.
First person shooters (FPS).
The original Doom - to see where it all started Quake 2 - Same creator of Doom (Carmack) with cleaned up graphics Lara Croft series - crappy interface (no mouse) but fun story line, especially if you're using a 10 year old win95 system. The original Half-life - back when a game took months to play, not just a long weekend. Return to Castle Wolfenstien - Very good FPS with very smooth gameplay. Deus Ex - A freeform storyline with alternate gameplay based on the choices you make in-game. No One Lives Forever - just a fun 'female James Bond' game with great humor. One of my faves. Max Payne - another fun adventure. Introduced 'bullet time' which slowed down gameplay during heavy shootouts. Solder of Fortune - graphics that tried to gorify the art of body dismemberment with shotguns. Fun & simple gameplay Medal of Honor - AI team as you make your way through WWII. D-Day was a ***** to get through. Several others, but this list would keep you busy for several months.
Sim Strategy - Games that take days from your life Civilization 3 - Like most of sim games, a balance of production, economics, and war. Typical game takes a couple days. Rise of Nation - Like Civ3, but plays quickly from throwing rocks to slinging a-bombs in the span of about 45 minutes. Sims - Like playing virtual house. Typically loved by the women of the household.
Race games Grand Theft Auto - They're all good and currently highly controversial. Weak storyline but fun gameplay. My fave was 3 Mafia - similar to GTA but a true career path storyline where you start as a driver for the mob in the '20s and move up. Need for speed series - Emphasis on the ricer movement where you take a 15k honda and make it worth 100k by winning races. Re-Volt - a race game where you control remote-control cars and the house is your racecourse. Up to 8 person multi-play.
Flight games. Airfix Dogfighter - similar to Re-volt but with airplanes. Can be multiplayed with 1 opponent Crimson Skies - dumbed down version of complicated flight sims that typically make you memorize every key on the board. Longbow - a simple helicopter sim for your new joystick.
Roleplay - dungeons&dragons type stuff Warcraft, Guild and others have been mentioned by others. Personally, I like Dungeon Siege. Morrowind - singleplayer only. Another game that will take months to play as you build up your characters strengths and charisma.
LAN-party multi-play - My absolute favorite way to play. 2-to-8 close buds in the same basement setup - drinkin' and killin'. Serious Sam - absolutely no thought process involved. just shoot for 4 hours straight. Leading cause of carpal tunnel. Tom Clancy Rainbow6 series - Most fun I ever had playing. Several scenarios where your team must take out the terrorists without harming the hostages. Several variants to cooperative Vs Adversarial game play. Unreal Tournament/Rocket Arena and Quake3 - Us Vs Them in quick battles. The twitchy hand usually wins.
Can't comment much on on-line play. Have satellite and the 1-second lag makes for difficult gameplay.
Running a 2.53 Intel with an ATI 9600, 1Gig of ram, and, of course, 4.1 promedias on a soundblaster Audigy2.
Jazdale