Lenovo Thinkcentre MT-M9645-Q1U Need INFO

Hello i have a Desktop computer i have a few small questions about.

the Desktop is a Lenovo Thinkcentre (IBM) MT-M9645-Q1U S/N LKTXGG2

It is an Intel Core 2duo running windows XP

Intel(R) Core(TM)2 CPU
4300 @ 1.80Ghz 1.79Ghz
1.99Gb of ram DDR2
80gig HD SATA
USB2 Enhanced

the ram chips i am running are 1 gigs, i do not know if they are 4200U or 5300U or even 3200U it does not say on them. Just says 1g on them. they are a matching pair.

What is the best RAM chips to run in this computer ? 5300U or 4200U or 3200U ? also can i put in 4 Gigs of ram ? ? ? 2 gig sticks x2 ?

I am wondering can i put a video card in this computer ? Now it has a slot that looks like it holds a AGP card maybe ? but its White. it may be PCI ? but im not sure...

Any info you guys can provide would be wonderful. i am just trying to upgrade it a bit more for cheap. i dont have enough money to buy another Desktop.