No, what you want is this.
http://www.ati.com/products/radeonx800/radeonX800xtme/index.html
buy page is here
http://www.ati.com/buy/pricesmacusa.html
It has a Dual Link and a ADC port on the back, so you can hook up anything from a 30" Apple (Dual Link) to any regular DVI (hooks up to Dual Link) to any legacy ADC (plastic) Apple display.
It's also the fastest current card on the market for your machine.
Check out the ATI upgrade program, you might qualify for a discount.
They send you the new card, you send them the old. But it may be wise to have two video cards, one for backup as they are getting scarce.
3D gaming is always pushing the video card envelope, so it's actually best to get the best right away to prolong your 3D gaming experience.
If you "half-step" it, then when you use a new game, it's crippled.
3D gaming performance is almost entirely video card performance dependent.
See Barefeats comparison here
http://www.barefeats.com/radx800.html
I have the new Rollercoaster Tycoon 3, which 3D animates all the rides. I've created a park that's only 1/4 of the available space and I'm getting dropped frames on my 6800GT and that's more powerful that the 128MB ATI card you mentioned.
Quake 4 runs great on my card, Doom 3 is fine, just a few stutters.
UT2004 runs great. Nanosaur 2 is very smooth.
So spend your money wisely, get the powerful card. Trust me.