At 8 years old, to run Vienna, it probably will not work.
To answer some of your questions:
Assuming you are will be running just your DAW software and plugins on the master machine, you will want the slave to be the more powerful. It takes a fairly robust computer to run VSL libraries, even more if you decide to go with MIR or MIR SE (strongly suggested long-term. It is a great tool). Also, what sort of music will you be writing?
Personally, as for ram/CPU speed, I would rank them almost equally as you really need both. CPU speed determines to a great extent how much/how fast you can load, while without enough ram, by definition, you are limited. Hard drive speed doesn't matter as long as you stay with at least 7200 rpm drives. If you plan on running huge templates, then faster drives could help.
What libraries will you be planning on running? The four SE libraries can be run quite comfortably on a computer with 8 gigs of ram and a modest processor (even say an older Q9550). Long term, if you would be looking to add MIR SE, you would need 12 gigs of ram and an i7 930 processor. For use with many standard libraries, in addition to the SE libraries, and MIR SE, you would probably eventually want 24 gigs of ram, and probably a better processor. For full blown MIR, a dual xeon processor is the recommended choice, with a minimum of 24 gigs or ram.
Other factors to consider: will you be using VE Pro, or VI Pro? If using VE Pro, and planning to incorporate other company's sound libraries, having a more robust computer will help.
To summarize, the first step in determining the type of computer you will need, is to determine what libraries you plan to run on it. Then project out what you think you might want to add. When you have figured out your plans, then you can start to look at building the actual system.
The current Sandy Bridge processors are substantially better than the older i7's, but have one major drawback for DAW use: they are dual channel, and therefore limited to 16 gigs of ram max. Tri and quad channel Sandy Bridge chips are scheduled to be released probably in the 4th quarter of this year. As Sandy Bridge chips use a different architecture than the older i7's, a different mobo is required.
Good luck with figuring out your needs.