driving 3d game demo download3d Accelerater card for DUMMIES? Help!?

So I need to buy it in order to play games bc when I've downloaded demo of Tomb Raider Anniversary I understood I'm far behind without it. The good choice they say would be GeForce 6000 series / Radeon X series and the question is how do I install it??? Is this going to replace my current graphic card or it's add on? Will it be compatible with my system? Thanks for any help

* Processor: Intel? Pentium? 4 processor 3.00GHz with Intel? Hyper-Threading Technology
* Secondary-Level Cache (L2) : 512Kb advanced transfer cache
* Front Side Bus: 800MHz
* Memory: 512MB DDR SDRAM (expandable to 4GB)
* Memory Spedriving 3d game demo downloaded: 3200 MB/sec
* Hard Drive: 160GB 7200rpm (8MB Buffer) Parallel ATA-100
* Optical Drive 1: DVD-RW (4x/2x/12x) DVD+RW (8x/4x/12x) CD-RW (40x/24x/40x)
* Optical Drive 2: 52x CD-ROM
* Flash Drive: 10-in-1 Memory Card Drive( CompactFlash?,Type I/Type II, Microdrive?, MultiMedia?, SecureDigital?, Smart Media?,Memory Stick?, MagicGateMemory Stick?, Memory
If you know what a side bus is, you probably should have enough knowledge to answer this question.

You have not listed your current graphic card details, so if you dont have one already, it will be an addon..
If you already have a graphics card, the new card has to replace it.

driving 3d game demo download Geforce 6000 series is the best graphics card in the market.. if you can get that then your system will have more than enough resources to play tomb raider or any high end game
How is the GeForce 6000 series the best in the market when you can get an overclocked 8800GTX with 768MB dedicated GDDR3 graphics memory for AUD$600. That would be the best card out on the market but you shouldn't get any extra graphics cards until you upgrade your ram (at least 2GB DDR2 800mhz) and possibly to a dual core processor.