Yes, I believe you would be able to join in with your main machine.
If I was to build a dedicated server for hosting a game...
At least 1.6 GHZ Processor ( Go with AMD, and with Proper Cooling ) if you choose Intel, no lower then 2GHZ
Memory wise, I would have what I have now, PC2700 DDR RAM, 1024MB.
Graphics card, anykind, because you won't actually be playing the game on it.
You don't even really need a sound card.
HDD wise, I would go for about 80 GB with an 8MB buffer...