I would always go with nvidia, having known problems with ati stability for these apps in the past. Autodesk recommend the Quadro range for both products (
http://usa.autodesk.com/adsk/servlet/hc?siteID=123112&id=6711853&linkID=9240618 and
http://download.autodesk.com/us/qualcharts/2010/maya2010_qualifiedgraphics_win.pdf)
I've been using the Quadro FX1700 for a while and it copes well, but since I upgraded to AutoCAD 2010 I've noticed it starting to stutter on complex models, so could be time to upgrade.
Regarding bottlenecks, your general modelling isn't too GFX intensive. Most bang for your buck will be in a 64 bit OS on an i7 as I believe that's where the work is done, especially if you're planning on stunning renders for presentation.
You could throw £15k at a system like this, but if you're just starting then you can do well for a little under £1k. The difference between a £400 and £1000 bit of kit is staggering, but as with everything, to achieve the same performance leap again you need to add another 0 on the end!
You can do some fun things in Maya with a basic system, but if you're planning on using it even to half it's potential (and you'll probably find that once you've started you can't stop) then you'll soon run out of power, so go long.
Remember the rule of buying hard drives, work out how much space you think you'll need, double it, add a zero, and buy 2.