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$600 to $800 is plenty these days, so long as you aren't modeling every screw thread and always working with a rendered model with material and lighting styles.

Don't feel like you have to rush right out and buy a super expensive graphics card. Autodesk Inventor is certified on Intel HD3000 graphics and often performs rather nicely on the second-gen Core i_ chips with the on-die Intel HD3000 graphics, even with full robot assemblies.

My home setup built in July of 2011 - I had less than $800 into it at the time.
Antec One Hundred Case
500W power supply
Gigabyte Z68X-UD3H-B3 Motherboard
Intel Core i5 2500K Overclocked to 4.0 GHz
Hyper TX3 cooler
Western Digital Caviar Black 1 TB 32mb cache hard drive
Crucial 60 gig SSD caching drive.
G-Skill DDR3-1866 RAM, 8 Gigs
Running onboard graphics on the motherboard - no graphics card. Autodesk Inventor runs great, even while watching HD video on a second monitor through the onboard graphics.

My previous mobile unit was a Fujitsu LH531 Core i5 2510m with 8 gigs of RAM. Again, Autodesk Inventor worked just fine, even with full robot assemblies. In Inventor 2012, there was a minor issue with the graphics driver though on this machine.

My current mobile unit is an i7 Macbook Pro, currently with only 4 gigs of RAM, running Windows 7 x64 of course. Haven't done too much CAD work on it so far, but it seems to run Inventor 2013 just fine. Call me nuts, but honestly I liked the 14" Fujitsu better. The Macbook keyboard and trackpad annoys me.

At school, we have a few different systems, all running Windows 7 x64 of course.
-We have some small form factor HP machines with Core i5 2500 processors and 4 gigs of RAM, onboard graphics.
-We have some mid-tower HP Core 2 Quads with 8 gigs of DDR3 1333 RAM and ATI FirePro V3800 cards running 2 monitors
-We have some mid-tower HP i5 2500 machines with 6 gigs of DDR3 1333 RAM and ATI FirePro V3800 cards running 2 monitors
- And finally, one mid-tower HP i5 2500 machine with 6 gigs of DDR3 1333 RAM and ATI FirePro V4800 card running 3 displays at 1920x1080.

All systems run Autodesk Inventor just fine for most purposes. We generally avoid working on the full robot assembly on the small form factor machines with only 4 gigs of RAM. There is just a little bit of lag when working with the full robot assembly on the V3800 cards. The V4800 card is a significant step up, with noticably faster performance on big detailed models. It can work with a full robot CAD model fluidly while playing HD video on two other displays. Neither of these are anywhere close to the best cards out there, but we had to outfit a total of 13 machines, and it's what we could afford at the time, and it suits are needs fine, except for the fact that I hate ATI Catalyst software (it does work though).

Really, if budget is a concern, I'd say you'd even be fine on a second-gen i3, so long as it was one of the better i3 processors with the HD3000 graphics. General CAD work, 4 gigs of RAM. Full robot assembly, 8 gigs. In terms of number of cores, it's only going to matter if you're animating and/or rendering. For most CAD design work, you're only running one thread on one core anyhow, so a dual core processor is fine.

One thing to keep in mind is that there's differences between gaming-oriented and "workstation"-oriented graphics cards. For CAD work, you want a "workstation" card and something that has been tested and certified with the CAD program you'll be using. Don't just assume that any card with good specs or reviews will be a good choice for CAD.