Help me with RAM overclocking

I'm a total n00b in overclocking but would like to fiddle around a bit to get the most out of my PC. (Well not really, just mild OC).

I've already done some OC to the CPU and to the GPU and am happy with the results but I'm a bit lost when it comes to the memory.

Here are the specs of my PC.

MOBO: DFI Lanparty UT SLI-DR Expert (Socket 939)
CPU: AMD Athlon64 3500+ Venice @2.2 Ghz (OC'ed to 2.4GHz)
Cooling: Thermalright XP120
GPU: Club3D Geforce 7900GS (OC'ed to 525/715). Soon I will update BIOS to 7900GT version.
RAM: GSkill F1-4000USU2-2GBHZ.

My doubts are that when I enter the timings in the BIOS I don't know if the changes stick or not. When I check the CPU-Z tool, the Memory tab and the SPD tab display different information. What is right?

The timings on the ram sticks are 3-4-4-8 but at what speed? DDR500? What happens if I lower the speed? Can I tighten even more the timings? Or is it better the other way around, loosen timings 3-5-5-10 and try to make them go faster like DDR520?

Thanks in advance if anybody can shed some light upon this.

Download Memset: http://www.tweakers.fr/memset.html

It will show you ALL the timings from Windows (and may allow you to change them from Windows). The best way to measure the performance change is with Everest or Sandra. If you had an Intel system, I would definitely say OCing RAM would improve performance. I'm not sure that is true with AMD.