I don't want to get into the business of recommending specific hardware. However, user experience has been that at least 1Gb of ram is required for medium to large size boards to avoid excessive swap file usage by Windows. Users with 2Gb of ram seldom have problems. Windows XP can't use more than 3Gb for user programs. So if you install 4Gb of ram, Windows will use 1Gb, and user software will use 3Gb. Note that this also depends on the motherboard chipset and bios.
If you run AD in 64 bit Vista, it runs under what MS calls "WOW", or "Windows on Windows" virtual machine because it is a 32 bit program. When running under "WOW", you still have a 3Gb limit because the 32bit program running in the virtual machine cannot access the 64bit memory controller or driver.
Nvidia graphics cards have proven to be the most reliable. However, the Nvidia Desktop Manager does not get along well with AD or a number of other graphics design programs. It is best to either not install it at all, or to remove it after installing the latest Nvidia drivers.
The graphics card MUST support BOTH DirectX 9.0c AND Shader Model 3 in order for the 3D features of AD to work. Nvidia graphics cards from GeForce 6600 models up through GeForce 8800 models have proven to work well. The newer GeForce 8800 cards give the fastest 3D response. If you are planning to do a lot of 3D work, it would be best to get one of the newer cards. If you don't mind slower panning and rotation in 3D modes, the older 6600-7950 series cards will work. Just be sure that the model you choose supports BOTH DirectX 9.0c AND Shader Model 3.
ATI graphics cards will also work for AD; however, there have been a lot of driver problems. Some cards and drivers work OK, but others seem to have random problems with screen refresh, panning, and unpredictable crashes. I don't have much faith in ATI graphics cards for any serious business application, but especially not for AD.