Ok, funnily enough, I was in the exact same situation as you.
After I upgraded my ram on my Macbook to 4gb, games saw a SLIGHT increase. But I'll let you know how these games run after I tweaked with their graphics settings, upgraded the drivers to the latest ones and even used steam commands to get these games to run as best as they could. This is how the games ran.
TF2- Will Run like CRAP and pretty much be unplayable, or will freeze/be choppy a lot. Can only run "decently" if you put everything down to bare minimum, rez at the lowest possible setting and forcing the game to run on -dxlevel 81
BF2 2142- will look like a movie out of the 70's but will run
PORTAL- will virtually die every single time you LOOK at a portal, you will have to turn the settings WAY down and possibly force it to run at -dxlevel81 or 80
Half Life 2- will pretty much run on all high but when there are many enemies on screen it will slow down, so turn settings down a bit.
Half Life 2: Episode 1- Lost Coast: Performance will start to degrade as you get to the newer episodes, but will run, just be sure to turn the settings down.
CSSource: Runs perfectly fine, but you will find a lot of artifacts and render issues (e.g purple glass or walls) but this game will pretty much run on all high.
GTA: I haven't tried it, but I'm confident it will run
Blizzard Games: Will run all Blizzard games currently released as far as I know, but with newer games like WoW it will need some settings to be turned down.
Steam Mods: Most of the mods I tried like Zombie Master, Hidden, Dip Rip, etc ran fine, but you had to tweak with graphics settings a lot of the time.
Keep in mind, never to really put on any anti-aliasing and to keep filtering to tri-linear in steam games. Also be sure to keep the macbook off you lap, It will literally burn your family jewels off

!
I honestly couldn't stand how much I had to tweak settings and stuff so I went and sold my MacBook to buy myself an iMac again, and have been happy since. Popular to contrary belief the BASE iMac will run all of those games at high just fine, people just love to underestimate the 2400xt a lot (whether it is due to a superiority complex issue with their own machines, they've been looking at too many synthetic benchmark results, or are just FPS whores

) , but in reality it is a decent card. I play games like TF2 on high and it runs like a champ! Just don't expect to run crysis on high
All-in-all your Macbook will run most of the games you want, with a little tweaking and as long as you up the ram. If you really can't stand it, I suggest selling it and buying a new iMac, or waiting it out for a new Macbook or possibly a refurb Pro.
Hope this helped.