I am working in a mixed PC (4+) Mac (4+) environment in which we are running Retrospect 6.0/6.1 on an old eMac (~ 900 MHz, 1GB RAM) to back up both the PCs and Macs onto an NAS. Needless to say it is very very slow and will take a week to recycle/backup even one single desktop of 160 GB.

I am either going to be getting a slightly used 20" iMacs desktop (Feb. 09 version, 750 GB HD, 4 GB RAM) or purchasing a whole new system. I would like to use this desktop or a new Mac Pro as both my desktop computer in the office and also have it back up all of my PCs and Macs. I am hoping to find another backup program other than the new Retrospect since I have heard people have nothing but problems with it and I can't afford that since the PCs are all analytical instruments with critical data.

I would probably like to keep the NAS (RAID 5 Terastation, 1TB) offsite as one of my backup sets but am also thinking of setting up a local drive to be my primary backup (probably something similar in terms of HW with either FireWire or USB2.0)

1) Is it reasonable to think that the iMac is going to be able to handle this and still function as my desktop?
2) Will it be considerably faster than the current eMac dedicated to this and be able to recycle a 160 GB backup set in less than a day?
3) If I am kidding myself on the above 2 questions, would the Mac Pro (single processor) be fully capable of this or would I have to go to the dual processor so as to be both fast AND still function as my desktop computer?

4) A further complication is that the PCs are on a closed intranet (using appropriate IP addresses) while the NAS and my new computer and other Macs would be on the internet. Therefore either I would have to use two ethernet cards (which would preclude the use of the iMac, since I can't add a card and using USB/ethernet adapter would slow things down) or figure out some software/firewall install that would effectively isolate the PCs on a 'virtual intranet".

Any suggestions, comments (either flaming or whatever) greatly appreciated.