Couple of websites regardless .net or django can be handled quite well with computers that can't run a Vista. Real question is how much load those websites are going to get.
I recently have been through similar problem, my solution was,
Server: Dual Processor Quad Core Xeon 5520 - 2.26GHz (Nehalem) - 2 x 8MB cache w/HT
Ram: 12 GB DDR3 Registered 1333 with Raid 1 2x 500 SATA
I will install XenServer on it (it's free), and run websites in VMs so, if you see one website is doing good or other is bad, you can allocate resources accordingly (give one VM more Ram, take from other) You can put your MSSQL on another VM etc.
Important consideration is getting the Nehalem processor, as it is recommended by Xen. Of course you don't need to buy so much ram that I bought, we have 2500 light weight websites running on the machine, and for those 4GB Ram is sufficient. Rest is for our other servers like mail, mysql, etc.
Without virtualization, most of your resources will be wasted on a single machine, and backing up, monitoring tasks will kill you. Of course, if your site is getting millions of hits a day, that's totally different story. I assume you are not facing that problem yet (otherwise you'd be looking for a cluster of servers instead of one).
Hope it helps.