I did a bit of looking around. If you're really intent on getting what little more you can out of the system...
The best AGP card I could find was the 6200. Not a huge step above what you've got, but it's still better. NewEgg.com has a number of them:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600030348%20600007850%20600007303&IsNodeId=1&name=GeForce%206%20seriesYou could also go with one of the ATI cards from the 4000 series, they made a number of these for AGP as well:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600030349%20600007850%20600007317&IsNodeId=1&name=Radeon%20HD%204000%20seriesA video card is going to cost you between $24-$70+, depending on what you want.
As for memory, another gigabyte for that will be about $45. Going over 2GB for Windows XP is overkill. Don't waste your money. So, for about $70, you could upgrade both, or go all in for $110 and get the best you can. Now note that these are prices for NEW parts. You could go for used parts for a fraction of the price, but there's not telling how reliable those will be.
As for what BC suggested, that is also a way to get a bit more out of what you've got. Regularly performing the things he's suggested will keep your system running as smooth as possible. As he said, disk checks, virus checks, reducing the number of programs that come on at startup, in addition to regular defragmentation, stopping some background processes, closing all unused programs while you're playing games, turning detail down a notch in-game (I know, I hate doing that too)... that, combined with a new card and more memory might give you a boost, but I still don't think it will be a whole heck of a lot.
Oh... got sidetracked by his later post... his first post definitely entails everything you mentioned. (I was thinking that they <only> wanted to make CS 1.6 faster).
No worries mate. It's not like I haven't done the same.