Hiyas, Jamie,
This is a 3 step process:
1. determine what type of new graphics card your computer will support.
2. Choose between NVIDIA and ATI chipsets for the graphics card. (just buy NVIDIA).
3. Based on your budget, buy the fastest NVIDIA card that will fit your computer.
STEP #1 ===========what graphics cards can you use?
I can't tell from your Forums description, but I *think* your computer has a PCI-E16 connector. You should try to confirm this before purchasing a new card--saves the hassle of returns if it doesn't fit.
IF you can post the make and model of the computer, and what year you bought it, we can figure out what sort of graphics cards it can use. Otherwise, here's how to figure it out by visual inspection.
-----------------------------Current computer----------
Based upon the forum information, your 'ATI RADEO Xpress 200' graphics are probably built in to the main 'motherboard' of your computer. This means there's no existing graphics-only board that needs to be removed. There are probably NO extra cards installed in the connector slots in your PC.
-------------------------checking for PCI-E16 vs. AGP---------------------
REmove the cover on your PC and look inside at the connectors.
Add-in cards, such as for a new video card, plug into these socket connectors, standing 90degrees from the main circuit board.
Here are some pictures of the types of connectors you're looking for:
PCI-Express connectors:
http://en.wikipedia.org/wiki/Image:PCIExpress.jpg(from the top, PCI-E4, PCI-E16, PCI-E1, PCI-E16, and old PCI.)
AGP-connectors:
http://en.wikipedia.org/wiki/Image:AGP_slot.jpg(from the top AGP, and 2 old PCI connectors)
If you look closely at the pictures, you'll see the printed labels of the components. (this is clearest on the AGP picture, where the connectors are labeled at their upper left corners.)
----------------------------
Look at the shape (not necessarily the color) of the connectors in your PC.
The big differences are
-Length of the socket,
-location of the little 'key', or 'solid bar in the middle of the card slot'
-and the little plastic bits on either end which hold the cards in place.
Ideally, you will find a long PCI-E16 connector. Any printing on the main PC circuit board should match your analysis. If not, message back.

An additional article with more pictures of what to look for is located here:
http://www.viaarena.com/default.aspx?PageID=5&ArticleID=383------------------------
IDEALLY, you have an unused PCI-E16 connector! This is the current speed champion. And the video cards currently available often use PCI-E16. Prices are low for these connectors.
STEP #2 ===========NVIDEA vs. ATI
just choose NVIDEA. You'll not get strong arguments against this choice. Many folks are unhappy with ATI and SL.
STEP #3 =========== Find the best card available for your budget.
Both ATI and NVIDIA make the component chips,and then 'retail' companies put the chips on boards, package them, market them and resell them. So, you'll find MANY brand names for a particular graphics card. But in every case, they will market the underlying chipset maker and model number.
Online retailers such as 'www.NEWEGG.com' sell many brands of both ATI and NVIDIA based graphics cards. So many, that it can get very confusing.
See the table on this page:
http://www.hardwaresecrets.com/article/132the slowest cards are at the top, and they generally get steadily more powerful as you go down the list. Simplistically, the KEY statistic in this table is the 'Pixels per clock'--the larger this number, the better.
Within families, you may find styles such as 'GS', 'GT', 'GTX', etc.
These styles reflect only minor enhancements. (Before spending extra on them, see if the next up model's basic version isn't cheaper).
Be sure that the card you pick supports DirectX 9.0c and OPEN GL 2.0 or better.
More memory on the card, up to 256Meg is good for a noticeable performance boost. 512Meg is better, but pricey.
The 7600's seem to be good value. Newegg.com has one on sale for $65 USD after rebate. If you find a good deal on a 7800, that'd be even better.