Be aware that flashing a PC graphics card to work on the Mac is piracy, just like downloading a copy of Word of Photoshop. I don't want to debate whether or not it's true - it is, read the license agreement for the software. The EULA doesn't leave a whole lot of room for argument.
If you're cool with that, fine. I ain't gonna stop you. But please be aware of what you're doing.
On to more interesting things....consider the WD Special Edition drives too. They're fast + have the 8MB buffer. Also, consider maxing the RAM. Depending on what you're doing, there's a big difference between 500-600MB and 1GB+. We all know how OS X loves memory...
Finally, +1 on not spending more than 50% the cost of a new machine. A new machine will get you a faster bus, better graphics, bigger drive, support for large (>137gb) drives, etc to start with. You could sell the Sawtooth on eBay for a fair chunk of cash. Put that money together with the money you were going to spend on upgrades and you might be able to snag an dual 1.25ghz....