SSD are coming down in price a lot recently 240GB for about £150 (UK pounds) (cheaper than 180GB version ??)
120GB for about £80/£90
This is enough to install windows & programs BUT not your data so you also need a fast hard disk.
SSD will reduce Windows startup time significantly.
Though once Windows is ready to use may not be that much faster in CAD program
Some one with a old version of Autocad was asking if it would work under Windows 7 or Virtual XP in Windows 7
( a good reason to get Windows 7 professional rather than Home)
also will it work with Windows 7 (64 bit version) if not using Virtual XP.
list of minimum and recommended hardware/softwarehttp://usa.autodesk.com/adsk/servlet/ps/dl/item?siteID=123112&id=15412189&linkID=9240617
Graphics cards suitable for CAD are not the same as those used for Games
Also a lot depends on what sort of screen you are using.