Hm, your system specs don't seem to be too far off from mine (though I'm sure your video card is more powerful than my Mobility Radeon HD 3650).
I'd estimate that the most popular games in ToJ right now are: TF2, World of WarCraft, Star Wars: The Old Republic, Left 4 Dead 2, League of Legends, and Minecraft. We have active chapters for other games as well, but those are the chapters I'd mark as the most popular at this time.
StarCraft II is easily the most popular game in ToJ (and the only RTS for which we currently have a chapter) and SC2 chapter leadership is planning on starting a regular game night soon.
There are also "flavor of the week" games that people purchase on Steam, play for a week or two, then return to other games. FotW titles include: Magicka, Dungeon Defenders, and whatever MMO just went free to play (Champions Online, DC Universe Online, Dungeons and Dragons Online, etc.).
You may still be able to round up other ToJ members to play those games (I'd be interested in playing Magicka and Dungeon Defenders), but it's not a sure thing like those chapters with guilds or game nights. Note that not everyone loses interest in FotW games, but the interest in such games tends to drop off after 1-2 weeks. Sometimes DLC or updates will reinvigorate interest in FotW games for a few days.
Guild Wars 2 is sure to be popular when it's released and will feature the same pay model as GW1 (i.e. periodic expansion packs and no subscription fees).
If you like YAMS (Yet Another Military Shooter) games, Battlefield 3 looks shiny. Unfortunately, EA refused to publish BF3 on Steam so you'll need to install and use Origin to play. It's not my thing and it hasn't yet picked up steam (pardon the pun) among ToJ members yet, but that could change.
Skyrim is easily the most popular single-player game among ToJ members (and the world) right now, but I don't know how well it would run on your PC. Unfortunately, no plans have been announced for a Skyrim demo and, given the open-world nature of the game, there probably never will be one. I'd strongly advise reviewing the game's recommended requirements (and, at this point, waiting on a sale) before picking it up. It's an excellent game (95 on Metacritic!) and I've enjoyed what I've played so far. I plan on waiting until I build my next PC gaming rig before continuing, but it's hard to stick to that resolution even though I have to run the game on Low-Medium settings on my laptop (and even then only get 20-25 frames per second).
. It should be up and running again by next week and now with Windows 7 64bit instead of XP. I'd like to join in some of the fun again soon. Of course Unreal Monday is a given 
.
