Exactly as Dale says buy the Mac pro with minimum ram like 2 gb , and the cheapest hard drive. What you are after at Apple is a good processor and that is about it. Everything else you build from there . Video card maybe good from Apple but we have to check around on that one. BTW buying the fastest processor only get's you so far it really comes down to drives and ram. As Dale said OWC for the Ram it is cheap and Newegg is great on hard drives. Unless you want the new 300gb 10 k drives which I have 3 of them now.
here is a simple config at Apple that maybe just enough
Part Number: Z0EM
Two 2.8GHz Quad-Core Intel Xeon
2GB (2x1GB)
500GB 7200-rpm Serial ATA 3Gb/s
ATI Radeon HD 2600 XT 256MB
One 16x SuperDrive
Apple Mighty Mouse
Apple Keyboard + User's Guide
Accessory kit
Gift message: Add
Remove $2,849.00 $2,849.00
Than with two 2 gigs on board add 12 gb of Ram from OWC for a total of 14gb if you want to stay on the low end of price. 279 for 12gbs
http://eshop.macsales.com/item/Other World Computing/64FB2MPK12GB/
Now i have three of these and my machine flies. Have one for the OS than have a second one partitioned like 100gb for Photoshop scratch and the other 200gb for temporary video or stills as a working drive. So edit off this drive and do your work.
WD VelociRaptor™
SATA Hard Drives
300 GB, 3 Gb/s, 16 MB Cache, 10,000 RPM
3.5-inch (backplane-ready)
http://eshop.macsales.com/item/Western Digital/WD3000HLFS/
Than fill the box with 2 1TB Seagates or Western Digital for storage there not expensive. Look for something like this 1.0TB Seagate Barracuda 7200.11 SATA II 7200RPM 32MB . Key words 7200 rpm and 32mg cache. I have these
http://eshop.macsales.com/item/Seagate/ST31000340AS/
The video card I just ordered this one
http://eshop.macsales.com/item/ATI Technologies/100435928/
Now here is where we would have to see what maybe best for video. Which is a little different animal than stills.
But I would go with a 2.8 or even the 3.0 and build it outside Apple. BTW i love my 30 inch Apple Cinema but watch out here this could be getting a update early next year but even still great monitor and still reasonable in price unless you went Eizo which are great but pricy. Just some things to figure out and look at in the pricing area. But Ram is dirt cheap right now
Others may have some different thoughts here also on final setup of drives. Also the drive you buy from Apple after adding the others you simply may want to use for Time machine in a Firewire box. That is what i have done