Servaasd

Hello, I'm new here and I'm a huge rookie with hardware upgrades etc. That said, here's my "problem". I'll explain as good as I can... here it goes:

I will start of with my system specs:
AMD Phenom™ X4 9500 quad-core processor
3 GB DDR2 Dual Channel RAM
500 GB S-ATA harddrive, 8 MB Cache, speed:7200 tpm
NVIDIA® GeForce® 9500 GT / 256 MB DDR3 SDRAM

Past Problem:
In the past, I tried to change my videocard, but the new one didn't fit my rig... that's when I realised that I don't know anything about pc 'modding'. I need help with this. I don't want to make the same mistake twice.

New Problem:
I want to change my Geforce 9500GT with a GeForce 9800GX2.

Concernes:
- Powersupply: The GeForce 9800GX2 uses more than 500 watt. Now, I don't know what my powersupply is right now. Does anyone know what the powerconsumption is of a Geforce 9500GT card?
- Bus-type: The GeForce 9800GX2 uses PCI Express 2.0. I thought I read somewhere that the geforce 9500GT uses this too. I need confirmation on this.

Sollutions:
- Can someone help me with powersupply-thingy... I absolutely don't know anything about that. Where can I find the powersupply capacity that I have know? Where can I find powersuppliers, what sort does a desktop use?
- The Bus-type is okay I think, just need confirmation from one of you guys.
- Are there other things I have to worry about? Please tell me.

Thanks in advance