Help a guy out here

[I Hate Pants]

Alright I think i've asked these questinos before. But I wanna make sure I know what im doing.

Firt off will I see a noticable fps increase in PS if I upgrade my ram from 512 to 768?

Secondly on the side of my computer it says this:
"256mb SyncDRAM Total System Memory
Upgradeable to 512mb"

Does this mean: (a) I can only have a max of 512mb of ram in my computer or (b) both of the slots my ram goes into (i have 2 slots for ram on my MoBo) each be upgraded to 512mb, therefore allowing me to have a max of 1024mb of ram?

And lastly I have currently a 1.2 Celeron processor. If I upgrade to a Pentium 4, will I see an fps increase?

Keep in mind I want to run PS on the highest graphic setting as possible with minimal lagging.