Upgrade Issues

I've been given a PC by some friends of mine and i wished to upgrade it, could anyone please give me some advice?

The PC is a Compaq 510 small form factor (also known as D51S). The PC is a P4 1.8GHz with 256 mb RAM, i've checked the specification documents and apparently the PC can hold 2Gb of RAM in any variation (ie: 256 + 1024 chip would work). However the type of RAM is DDR2100, i've checked the crucial website to see what RAM they recommend for the PC and its something like DDR2100 and DDR2700 - but the higher spec will only run at DDR2100. It didnt mention the faster DDR3200 RAM. Rather then buy out of date RAM, i would rather buy the later DDR3200 incase i then get a newer PC so it can be reused. I would of imagined that Crucial would of just assigned what RAM they have for that computer at that time, and not updated it if newer compatible RAMs become available at a later point.

So what i am asking is whether the PC will run the DDR3200 RAM at the slower speed or whether it will just not work all together? If its a yes it will run it, how certain are you?

I also wanted a new graphics card, i dont know whether the PC has a AGP8x slot or a AGP4x slot, is there anyway to tell this easily? Also do you know of a particularly good graphics card for this system..... by good i mean good value for money

Thanks for any help!