Some additional info is needed.
Are you upgrading 1 server or several? Keeping existing hardware or installing new?
Do you already have GW and Bordermanager? What versions?
How many users are you supporting?
For the most part, if thehardware is relatively new, you can put NW6 and etc. on it. The newer software is more RAM intensive -- 512MB is really the minimum to start with for NW6; 1GB may be what you need.
Having BorderManager and GW on separate servers is also a good idea. Since server licenses are essentially free under NW6.x, I'd plan on it.
If you are putting in at least one new server, check out the Migration Wizard from Novell. it actually works pretty well.