I'd rather steer clear of Nahalem and focus my search on Westmere - this is where the raw power at the moment resides. I believe you can find some year old second-hand tower at affordable price. And look only for dual cpu architecture. Anyways it's your wallet and it's up to you to decide what price is affordable and reasonable.
BTW, I too have MacPro 1,1 and many argue that it's the best Intel mac made by Apple ever since they shied away from PPCs. Everything that came after was 101 marketing. I still hold down to mine dearly.
Production and Postproduction in Moscow
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