I also wouldn't worry too muich about graphics cards choice. Signal differences (at least with digital output) are no longer of any concern between the various models. I would give more attention to things like silent operation and sturdy build. Usually cards made by Asus are quite good, Gigabyte and MSI too.
As to whether Nvidia or Ati, well, that depends. Ati has a long history of not so reliable drivers, Nvidea's been better in that respect. Both had problems getting out decent drivers for Vista at the beginning but now they're good (if you don't use SLI or Crossfire, which can still be a little tricky: Some have no problems, others are going mad). To be on the save side get the WHQL drivers and not the latest uncertified drivers unless they fix a problem you experience yourself.
Any card with more than 64MB video ram should be sufficient for 2d tasks, so it doesn't matter that much which GPU you're using for photo editing because these tasks are performed by the CPU and the CPU needs system RAM. The graphics card's only there for diplaying the output.
Of course, if you're doing things like video encoding or playing back HD video content then you should look for cards with GPUs that support these features. Ati has a slight edge here at the moment, but that changes about every three to six months. Could be Nvidea next time.
Now, if you're also going to use Photoshop CS4 then it's getting interesting. Some tasks can now be done on the GPU and here it's true: faster is better. These effects are OpenGL based and here Nvidea has traditionally been somewhat faster than competing ATI (ok, AMD now, sorry, can't get quite the hang of the new name ) cards but AMD has come close, sometimes even overtaking Nvidea (depends on the application). You don't necessarily need top-of-the-line Quadro cards. These cards are usually almost identical to consumer parts regarding the GPU. They've got more video RAM and special drivers that can be used by applications like 3dmax or AutoCAD and some can control more displays at once. But from a performance point of view traditionally (save in said applications) they're not (or at least not that much) faster than their consumer counterparts. But you'll be paying a hefty premium over the consumer cards.
I don't know how much you want to spent, but I think a 1GB card such as a Nvidea GTX 260 (Core 216 is a bit faster) or a AMD 4870 gpu would be best from a price/performance ratio. They're about equal in performance.
If you do not use PS CS4 an AMD 4670 based card (preferably passively cooled) would be a good alternative since it has decent video accelaration. But here it depends how much such are card costs and faster models might get interesting (AMD 48xx, Nvidia 98xx, 9600GT). Depends on the current pricing.
Of course, if you can afford it and use PS CS4 THE card to be bought would be Nvidea Quadro CX based. Basically the same as a GTX 260 but with more support for things such as video encoding in Premiere or accelerated RAW and DNG decoding. Here in Germany such a card starts at about 1700 € (GTX 260 Core 216 begin at about 260 €).
So, your choice