the actual diffrence between cheap and expensive memory is the timings, and how far it will overclock (and sometimes, stability) - cheap DDR400 might run at cas/cl 3 (spectek etc), budget branded ram will do 2.5 (corsair value seletc etc), and the expensive stuff will do cl2(corsair xms etc) (lower the less time between cycles or the faster the system will respond) - the diffrence? price, and 5% performance.
You'd do better to replace the lot (slap those sticks in a friends system!)with 2 ea 1 GB sticks, preferably with Corsair XMS or Value Select, depending on your budget, as running them together would cause the mem to run at the timings of the lowest quality installed....
In gaming, there might be a 2-3% difference between CAS2 and CAS2.5 in some framerate benchmarks, but then the gpu is the most important there anyway...
Im thinking I wont be replacing the Graphic cards soon, as I only bought them in January, but I do think the move from 1GB to 2GB will about set my hardware for this year! eBay will benefit from the old stock me thinks!
Although some added cooling might be a consideration in a few months!
Watch this space!
Thanks mdd1963, XMS was sort of where I was thinking.