I would say the problem is ram: 256 is nowadays simply not enough ram for 3d-gaming.
i would suggest you buy urself 2x 512mb of ram and let them run on dual-channel.. that way they can process information paralelly. (if that mb supports dualchannel)
If you can afford it 2x 1gb would be even better. But make sure you make use of the dual-channel procession if possible.
It's not too expensive anymore.
for example: I am still using an AGP-System, not PCI-Express etc.
I have an AMD Athlon XP 2800+ so your CPU is not the bottleneck.
I have a Geforce 6600 GT passive cooled, which is not new either. But i use 2x 512 mb ram running in dual-channel.
I can play with about 20-30fps.. even with a moderate numbers of players in gunmod. (Gunmod Rocks!)

But regarding choke thats another issue once again.
There are many *****s out there who use some lousy server @ Tickrate 100 setting for supposedly meeting useless clanwar-standards. But if the server is not too good.. it actually makes things worse.
If you do not have any tremendous choke issues i would keep the settings as they are. If you have trouble google for "best net settings cs source".
There are different ressources, but only one or two of them are really good.
After all this will only improve your Ingame-Connection. I strongly suggest you upgrade those Rams.
And maybe you could still find out what GFX-Card you have by looking at the advanced desktop properties.