The fact that Ubuntu uses less system resources isnt going to make a game run any faster unless the game is maxing out your ram. This is because once your ram is full your system uses the swap file in windows and that slows things down. With games like TF2 and CS:S, or DoD:S, 2 Gigabytes is more than enough to run with in windows, even windows vista,.
The main thing you'll notice is a decrease in load times, but nothing will change as far as the actual performance of your machine on the given game.
As these guys have said Linux is difficult to work with most games, it wasnt really made for games, but it DOES work, and there are plenty of Walk-throughs for specific games on the Ubuntu forums.
The format partition sizes-------
No you cant resize a partition without ruining any files contained within said partition.(unless theres some magical program that i dont know about yet.)
And in order to add a partition to your existing C: your going to need 1 of 2 things
1. If your existing partition doesnt use ALL of your disk, then you can simply stick a second partition in the remaining disk space and your good to go, but i imagine its using 95-98% of it already.
2. Reformat the disk and put a few partitions on it.
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If your going to get another disk just for Ubuntu, dont limit yourself with a crappy old one, get a decent one at a local Computer store and make sure to put 3 or 4 partitions on it when you first format it so that you can try different variations of linux and windows.