Yeah. If I was doing it in two boxes, that's how I'd set up the partitioning. Maybe vary the swap size depending on system memory, too. If you can, set up redundant drives, too. Unless you're running a mail server which is expecting heavy or rare use or is going to be running IMAP (I'm running an IMAP server myself, and you'd be surprised at how much mail storage a packrat will use up. I've got a 110mb mailbox, and one of my users is using 190mb at last count), you probably won't be filling up that 20GB hard drive any time soon, and it might be a good idea to set up a different partition where you can periodically back up the mail directories, especially if you're running an IMAP server. For mine, the backup is actually to an NFS-mounted share running on a diffrent computer.
It's probably worth asking how much use you're expecting to see, too. Both my web and mail servers are running off the same box, and the load has never reached anywhere near enough that I would actually need to split them off to separate servers. For reference, I see about a thousand hits a day on the web server, mostly remote-linked pictures from assorted messageboards I visit, and it's a PII-350 w/ 196MB of RAM, and /home is on an 80gb drive. You might find it's more worth your while to set up a single system with a 40GB hard drive (say using JBOD or some other spanning method), rather than two systems with smaller drives.