Dell Optiplex gx620, RAM limits

The manual indeed says 4gig of ram. A bios update may have allowed it to see 8 gig, not sure. Anyhow, XP 32 bit will only see ~4gig of ram. You need a 64 bit OS to see more than 4 gig.

Is seeing the same as using?

For example, if I were to play a RAM intensive game, would it use only the 4gb seen by xp or the entire 8gb I physically have installed?

Also, another follow up question that's probably stupid, will a OS upgrade to 7 allow programs to use all the RAM installed or is the limitation set by the motherboard?

Thanks

It will only use the 4 gig. As stated, you need a 64 bit OS to be able to use over 4 gig.