Since AMD cpus have the ram controller built in, perhaps your Sempron only supports 667Mhz max? I couldn't find this info anywhere, AMD really needs a similar processor spec finder as intel.
That makes sense, it seems that there is some issue with the memory controller here. The boot screen reports that the memory is 64 bit on the single 2gb stick and 128bit on the dual 512Mb stick memory configuration. So there is something happening there but apparently not everything that needs to happen.
I'm considering getting a new processor. There are some real cheap low power dual cores out there....
I tried benchmarking memory when I got my Athlon64. Sure, dual channel boots faster and shows better memory benches, but real world (feel, non-memory benchmarks) it made no difference. With integrated memory controller, the only real factor is processor speed (which determines on chip cache speed).
Until you run out of cache (or, yikes, have to swap).
The Prescott with shared video ram is a whole different animal.
As a guess, since you can get decent performance now, either of your upgrades would be better. I'd save up for a new mem/mobo/cpu and keep my eyes open for old machines to scavenge from.
Dual channel makes a *huge* difference only if have onboard graphics which uses system memory or a graphics card with a small amount of VRAM (i.e. 64MB).
The difference gets bigger as you increase the resolution of the games/apps you use. If you have a 32MB,64MB or even a 128MB card, you can also increase the GART size in the BIOS to at least double the amount of your VRAM for even better performance at higher resolutions.
Dual channel RAM will also improve performance and reduce latencies if you use audio production software like Cubase,Sonar,Rosegarden,Ardour,Reaper,etc.