|It would help if you listed the RAM correctly & got the terms right. There is no "pc2 5800s" so I assume you mean PC2-8500 (aka DDR2-1066). "gdd2 800" isn't right either. The "G" means it's graphics memory & you're missing the "R", so either you're asking about GDDR2-800 (on a video card?) or DDR2-800. I'll assume you mean DDR2-800.|
If you pair up DDR2-1066 with DDR2-800, both will run at DDR2-800 speed (400MHz). Depending on your CPU/motherboard combo, 400MHz is either too slow, too fast or just right. We have no idea because you didn't bother to list your system specs.