New Video Card for Dell XPS 400

Hi!

I'm sorry if this question has been asked before, I could not find info on this.

I'm looking to buy a new video card for my Dell XPS 400 system (Now Dimension 9150 I believe). I know, I will build my next system

Currently I have the Radeon X600 (256mb) video card.

What is the best card that I can buy that will not be bottlenecked by my CPU?
1) I was thinking of something along the lines of 9800gt?
a) Or is that too much? 8800GT?
b) Or can I only use ATi cards?
2) Will they fit into the slot in the tower?
3) Do I need to get more power?
4) Do I have to get a fan?

Thank you very much!

My Specs:

Computer
Operating System Microsoft Windows XP Professional
OS Service Pack Service Pack 3
Internet Explorer 7.0.5730.11


Motherboard
CPU Type Unknown, 3000 MHz (3.75 x 800)
Motherboard Name Dell Inc. Dell DXP051
Motherboard Chipset Unknown
System Memory 2048 MB
BIOS Type Phoenix (01/08/07)

Display
Video Adapter ATI Radeon X300/X550/X1050 Series Secondary (256 MB)
Video Adapter ATI Radeon X300/X550/X1050 Series (256 MB)
Monitor Plug and Play Monitor [NoDB] (T61165A4BAK6)

Multimedia
Audio Adapter Creative SB X-Fi [NoDB]

Video Adapter Properties
Device Description ATI Radeon X300/X550/X1050 Series Secondary
Adapter String ATI Radeon X300/X550/X1050 Series
Chip Type RADEON X600 Secondary (0x5B72)
DAC Type Internal DAC(400MHz)
Installed Drivers ati2dvag (6.14.10.6844)
Memory Size 256 MB

Video Adapter Manufacturer
Company Name ATI Technologies Inc.