Installing Windows 7 Ultimate?

I ran Windows 7 Ultimate with 1gb ram and a 2.6ghz Celeron D just fine before. Of course a Pentium 4 is better. If yours is anything under a celeron, I'd say upgrade. Otherwise don't waste your time. But seeing as you had XP previously, I can only guess you have a P4. Look up peoples computers running ultimate on your specific processor and ram for a close estimate of what you should get. (p.s those system requirements are off sometimes)