Which card should i use?

Hi, thought I'd best post this here...
I've managed to develop a new trade secret, involving a very basic (LOD200) house design placed on a life size decal of the surrounding area (coloured infra map). Finishing touch involves trees and vehicles etc. This always manages to impress a potential (professional) client, and gives a solid idea of how it's going to fit into the surrounding area.
Only problem is you're talking about an image of several megabytes which is then scaled up a several dozen times. From a bird's eye view (about a hundred feet away) it looks really neat. But it takes from 15 up to 30 seconds for a Realistic view to regenerate.
We work on Intel Xeon PC's (3.06ghz) with 8GB of RAM, and an NVidia Quadro FX 580 graphics card (512 MB). I suspect my graphics card is the bottleneck, since decals are supposed to be stored in gfx memory (am I right?)

So my basic question is whether a better graphics adapter would increase performance, and what price range am I looking at if I wanted to see some dramatic improvements? (anything less than 10 seconds for above scenario would be awesome!)
Hardware acceleration is enabled, AA disabled, and I'm using driver version 8.17.12.7642 (not entirely sure whether it is the WHQL driver, but overall performance is adequate).

EDIT - Some more info about the decal. It is typically and ideally a 30x40cm (A3 page format) PDF which I convert to a 600dpi PNG file, then load as decal and scale up to the actual project size.

Inspiration from the ever awesome Tom's Hardware page

We use the ATI 7850 in our machines, partly due to power requirements and the fact they are perfectly fine for Revit / most CAD applications requirements.

I can't remember with Nvidia if they have this functionality or not, but with the ATI control center you can look at the performance monitor for the card - i.e. GPU activity, memory useage, etc... to see how it's being worked and if it's the bottle neck.

With windows 7 task manager, under the processes monitor you will want to add Peak Working Set memory from View - Select Colomns, this will show you the max amount of memory that Revit is using/was not just what its currently using.

Thank you for your reply.
While opening a view with such a decall it is fully using one of my four CPU cores. So my assumption was wrong. In addition, while using 3GB of available memory at that time the peak memory for the process climbed to about 1500MB. I've tested the same operations with 3D support switched off and nothing seems to change, except for how ghosting (overlapping or crossing textures) is handled.
What I did notice is that my GPU utilizes 3500MB of shared memory so disabling that in the BIOS might change some things. Will check back if something unexpected happens.

You can pick up a Nvidia Quadro 4000 (2Gb) for $600 on ebay, or a Quadro 5000 (2.5Gb) for around $900. Each will supplement your ability to explore larger models in Revit (and 3dsMax if needed). The other thing you may want to try is faster RAM memory. What speed is your 8Gb RAM now? 667Mhz, 833Mhz, 1067Mhz? If you're at 1333Mhz, that's the fastest a Xeon X5675 (3.06Ghz) will take. Expect the prices of those two Quadro cards I listed to drop dramatically in the next 2 months as Nvidia is releasing their new Quadro series Kepler which blows them out of the water.

All that Xeon and Quadro stuff is a waste of money for Revit IMHO. You can do a complete upgrade on a machine including CPU, motherboard, memory, and graphics, for less than $900 using Intel Core i7 Ivy-Bridge 3.4 GHz processor, 16 GB of FAST 1600 MHz memory (less than $100 right now!), and 2 GB nVIDIA GTX 600-series graphics card. I guarantee you that setup will blow your current Xeon machines off the map in terms of performance.

Obviously, because my setup is two years old now. We were still unsure about riding the Windows 7 wave and I'm glad that I persisted while we had the budget. It would be daft to use XP these days on performance machines. I might keep your recommendations in mind tho when I'm building my next gaming rig, thanks.