Autocad will run quite nicely on even a 512MB or 768MB XP virtual machine BTW -- I'm running similar on a NETBOOK believe it or not. My XP VM is set to 512MB and it runs just fine under vmware workstation (vmware player also works and is FREE) on an ASPIRE one LO 521 Netbook
- I have 2GB RAM in the netbook BTW so your 4GB is PLENTY.
However I would DITCH XP mode -- it's a great idea but the overhead is quite high and the only advantage is that you get a "Free" XP system.
If you can source an older XP system -- perhaps from your previous computer where you ran AUTOCAD I'd install VMWARE PLAYER -- it's free and runs Virtual Machines very well indeed -- beats the SOCKS off XP Mode and with decent integration printing etc is not such a hassle to set up.
Also you could load the VM on to say a USB device and use the Virtual machine on ANY computer that has vmware player installed -- even LINUX machines.
BTW vmware player runs on ANY Windows 7 system including Windows 7 starter so you have much more flexibility in any case on where you run your Virtual Machine(s).