You should be able to do better than that with a 6600 in CS:Source.
Either another part of you PC is limiting you, or you need to change your graphics settings.
Also, if you really want to upgrade, "cheap" is too vague. We would also need to know some details about the rest of your system like whether it is an AGP card or PCIe card.
Not disagreeing with you at all about the fact that if the OP had another brain cell it would be lonely, but you do have to admit that CPU is the place where the "action is" on THG. I posted a question in the Software section over three days ago, and so far have three views and no replies. If I posted it here, I'd have 80 people chiming in, three flame wars, one post of the day and most definitely have an answer by now!