RAM
To calculate RAM required for a project add up:
4Gb for Windows 7
2Gb for Revit calculations (2 x Revit Programs folder size)
xGb for Project file (20 x Project file size + loaded links)
So minimum RAM for a 300Mb project is:
4Gb
2Gb
6Gb (0.3 x 20)
12Gb
Add in linked files adding up to 200Mb:
4Gb
2Gb
10Gb (0.3 + 0.2 * 20)
16Gb
GRAPHICS CARD
Autodesk recommend a card with 2Gb onboard memory.