I think there's a few components you could choose a bit smarter.
For the processor, since Inventor is still mostly single threaded, you benefit from as much GHZ as you can get, and 2.26Ghz is not all that much.
This E5507 is a cpu that does not have turbo boost technology, so It will never go faster than 2.26Ghz, and has no hyperthreading (where every core can do 2 threads), so can only do 4 threads (in Inventor 2012, only drawing view creation and rendering can do multiple threads)
Also, I believe this E5507 is a cpu that can run in a dual cpu configuration. You pay extra for this functionality for the processor and the motherboard, and is a waste of money of you only keep the 1 cpu in your workstation.
Instead, go for a single CPU workstation (like a Dell T3500), with a faster cpu like the Intel W3565 (4 core, 8 threads, 3.2Ghz, 3.46Ghz turbo boost), to give yourself a significant speed increase in just about everything you do in Inventor.
For your budget you might even be able to get a W3670 (6 core, 12 threads, 3.2Ghz, 3.46 turbo boost).
Harddrives are still one of the biggest bottlenecks in any system, so you want more speed there as well. The fastest would be to get an SSD as previously mentioned, but they're expensive. You won't need much space, so 120 or 160GB would suffice.
I normally spec similar workstations with a 300GB 10K RPM drive. Another option would be to get a 1TB 7200rpm drive. You won't need that much space, but the bigger the drive, the higher the data density, the faster it reads data.
As for the graphics, it's a neverending debate. For starters Dell doesn't let you configure a gaming card in their workstations.
Since Inventor uses DirectX and not OpenGL that's one benefit of the Quadros gone. Other arguments would be that Quadros have better driver support, run cooler and quieter, and are designed to run 24/7 for years instead of a few hours every night. I usually spec Quadro 2000 cards and the 4000 for people who deal with larger assemblies, and have been happy with them.
I would start with the following specs:
Dell T3500
3.2 Ghz W3565 4 core
300 GB 10k RPM
12 or 16 GB RAM
Quadro 4000
From that base bump the HD to an SSD first, if there's still budget left go to a 3.2 Ghz W3670 6 core or even a 3.46GHz W3690.
For the display I'd get two 23" 1920x1080 (U2311H) or 24" 1920x1200 (U2412M) displays.
I do find that prices quoted through our Dell business account are quite a bit cheaper than when you configure through their website, so what you can fit in your budget may vary.
