Generally speaking the answer is "yes." However, there could be a few catches. First of all, if your server is a DC, then you have to have DNS running on it anyway. So, I wouldn't bother moving DNS; if you're worried about load on the server, you could set up another DNS server on the domain as a secondary or additional DNS server.
As far as ISA goes, unless you are using any programs that require the ISA client to be installed, moving ISA should not affect workstations at all. They should be set to auto-detect the proxy server through IE, and that will find the new server. The only problem might be if the workstations have their default gateway set statically; that setting would have to be changed if so. If you are using DHCP to set a default gateway on the clients, you would change that setting and refresh the IP addresses on the workstations to ensure they don't fail to detect the new proxy server.
Hope this helps!