Upgrade advice

I have two friends with really old computers that are used solely for CS:S. We're all poor college students so money is an issue. Their setups currently run the game on minimum settings very poorly, they are hopeful of spending a small amount of money and getting a large performance boost. If I remember correctly one of the computers is a Pentium D @ 2.8ghz with 256mb of ram. I can't remember what speed the ram is and I've never used Intel systems so I'm not sure what his is. Probably DDR2-400? The other computer is a Sempron 3100 with 256mb of DDR-400. Both of the PCs have MX4000's which are PCI-E. I'm assuming the Pentium D is on a LGA775 socket, however it is possible it could be on the older socket(474 or something?) right? That is mostly irrelevant but if desired we could just drop in a Core 2 if its 775? The Sempron is most likely 754s which means upgrading the cpu is highly unlikely. I plan on sticking in a 1gb dimm into each and probably a 3850 or a 4830. After looking at the specs on the 3850 and the 4830 I'm unsure which is the better card. If the 4830 is better, is it worth the extra $40? More significantly, will those old CPU's even be able to utilize those gpus without posing as a serious bottle neck in regards to css? If not, would a 3850 or 4830 be over kill for css? If so, what is a better choice?(The availability of older cards has really gone down so finding them is obviously a problem) Is this most logical upgrade choice for $100-150?

Thank you for your thoughts.

P.s. No nvidia suggestions please.

Makes sense.

Thanks Proximon

Ram will be a plus.
For some of the other questions it would be nice to knoe more about the systems.

Sempron is fine with cs. It looks to me that this is mostly a ram issue.