Don't most modern computers use DDR2?

Depends on your definition of modern. Bleeding edge machines use DDR3. But the systems we're looking at here are most likely DDR2, yes. I don't know if we can rule out DDR entirely though.

This isn't 1999. Macs just use a limited array of standard PC hardware these days. Even the Mac Pro with its server mobo and processors still uses conventional DDR3 RAM.

If only Assassin's Creed had a gameplay as rich as its graphics... but that's another story...

The RAM has been the same since the dawn of time, or thereabouts. Hard drives and optical drives were SCSI for a long time (HDDs changing to IDE before optical drives did), and the only things that are different now are really graphics cards (because of PC's refusal to replace their crappy BIOS with OpenFirmware - apparenty that matters because the graphics card is started so early in the booting process), all other hardware merely requires the correct drivers to be available. Perhaps the evolution has even continued beyond my knowledge and graphics cards too are as compatible as all other hardware nowadays.

yeah... three years ago. now? nope. Mid range is 9800Gt or 4850.
Like other said:
CPU is perfectly alright. slower than other but it is duo. every game works with it including Crysis without problem.
The problem is 1GB of ram. It is really too little these days. get one more stick, maybe used one for like 30 bucks. It will improve the performance greatly not only in games but in normal everyday tasks.
Second problem is of course the card. 7600GT can handle quite few games (i know. i had it myself just till march this year) but it is a grandpa of videcards.
ram is a must.

Ok
I'll ask my local Mac dealer whther I can just stivk in any old DDr2, or I have to buy it
Thanks guys