1) Heapsize.
You gotta commit the ram it needs to run. 50% of your total physical ram (Not virtual memory) so 2gb means commit 1gb. Set this on the start up settings. I've even seen people commit 75% because once you are in game you aren't running anything else important so it can all migrate into the VM and be fine.

2) Config files.
These go both ways. You have to look at what settings you modifying. If you are running a 8800gt you dont' need to worry about Graphic settings (so water textures, etc. are fine) disabling those aren't going to do anything for your FPS. If you have a slow processor (really anything mono core under 3ghz or dual under 2.2ghz) disabling physics components helps a lot (no ragdolls or shells) . A note on rates/cl_updare, cl_cmdrate those are server set and don't mess with them cause it won't help on the cs:s d2 server 66,66, 2500 are the right rates for the tic of the server. Anything more will cause overflow. anything less will cause loss

3) Hardware
Hardware upgrades help tremendously to old systems. If you having FPS woes tell me your system specs a lot of cheap upgrades can help you. I went from a amd 939 3000+ to a 3800+ and jumped up 20fps. If you are on a geforce 6xxx or geforce X660 you need to upgrade. if you can spend $200 bucks go for a 8800gt from newegg (any manufacture is decent). You will almost for sure get 100 FPS at full graphics and high resolution. Just make sure your have a long case (the card is between 8-12 inches long) and a PCI-e connection. AGP graphics is garbage now.

4) Network connections
If you play on wireless you are going to have issues. If you don't have a high speed connection you'll have issues. CS:s sends a ton of packets back and forth and you need a very consistent large bandwidth connection. Turn off the limewire, bit torrent while you are playing the game cause it just hurts everything. Also portfowarding can help out a lot if you know what you are doing.