Just skimmed this in a hurry this morning. It seems pretty incoherent to me.
The PC3200 RAM will likely not be noticeably faster than the 2700, but it's a good price.
You can always find a use for the other RAM, or someone who needs it.
Your motherboard has 400 MHz FSB, which means it'll definitely take the 3200+ Barton.
There are no faster chips in that product line, so that's the limit of that board (and mine, BTW)
It will be faster in a noticeable way than the 2600 you currently have. Exactly how that translates to applications that matter (CS:S) is harder to say, and on top of that, only you can say how much improvement is worth that kind of money. I did it once, but wasn't playing CS at the time. FFXI ended up noticeably faster.
To respond to those who suggest you should buy a new athlon-64 rig, here's one reasonable way to think of it:
If you can't afford a new rig, it's not an option. At this point, it's nuts to dump money into anything that isn't PCI-X, which means that you're in for mboard, CPU, fan and vidcard, putting you up over $700.
If you plan to save for a new rig, it's perfectly sensible to make part of that saving plan the upgrade of your current rig to give it more life. The additional life you buy will give you some immediate gratification and make your saved $$ go much further than they would earlier. Test it yourself -- make an imaginary budget, go to newegg and price out some stuff today, then price the same stuff in a month and note that it's well over $200 cheaper than it was today.