I have two Win7HP, an XP Pro computer, FreeNAS based computer and an XP Pro netbook all happily communicating with each other behind a router. The XP Pro computer is accessed exclusively via the network, (RDP, FTP, etc), as it's headless.
The computers are all in WORKGROUP, (ie. standard for XP Pro and Win7), I don't use passwords on the accounts, (all single user machines), except for where it's required for Task Scheduling in which case those computers AutoLogon to the main user account at boot.
Static IPs, as wraith mentioned, but this is mainly so I can readily access them by typing in an IP without having the system try and lookup a name/IP relationship or having to edit host files on multiple machines.
The following settings applied to the Win7 machines:

The main one being the 40/56 bit encryption, I've found it makes the connection to XP computers more reliable, (but it does work with 128bit also - just seems hit and miss for some reason). The rest is personal preference, (username/account, public folders, etc).
On the XP side I use a few registry tweaks to make connecting to the machines a little bit more reliable/faster.
I can reliably just share a folder on a machine and expect to access from another whether it's 7->XP or vice versa.
And to follow up on what skwire just mentioned, I don't use mapped drives, network neighbourhood - just FTP or SAMBA.