I'd say your setup should work fine, either way. I have done systems with 1 GB Ram and the same processor and lower and the same Video Card and they worked fine. I have also never had a problem with the Composite Output, I have hooked them up to 21" LCD TV's and they looked fine, just a little stretched because the LCD's are widescreen (16X9) and the picture was set to fill the screen. The cable distances for the cameras should not be a problem if you use RG59 %95 braided copper and not some tin foil junk.
On a side note, we have done camera systems for Day cares and they actually offered the parents the service of watching the cameras from work (Multiview) for a fee every month ($5 or something), and they changed the password every month. It worked out great for them and it helped pay for the system. the parents liked it too and it increased their business becuase they offered this service. just an idea.