You could try AoE 1 or 2, both are good games, or C&C Tiberian Sun or Red Alert 2.

this

Dawn of War or supreme commander forged alliance. If you have computers able to handle it you can have some amazing multiplayer lan battles in supreme commander, the scale of that game is awesome.

It's not what I'd consider old school but you could try Rise of Nations Legends for a fantasy based take on the rise of nations games.

Universe At War would also be another consideration, presenting 3 totally different races to play (and by totally different I really mean that they are; they don't use similar tactics or build orders or anything compared to most RTS where different armies are just reskinned over the same basic concept). Ps early reviews of this game were exceptionally low because it was released in teh early days of Windows Live (when they charged you for mp! ) so it got a big beating the game itself didn't deserve.

Sacrifice - not RTS in the traditional sense, but rather more like a tactical squad controller with some limited base expansion.

Sins of a Solar Empire - if you want an RTS game that can (if set to a large map) take hour upon hour or intense and exciting battles to finish (and if one person has to leave the ai can take over their side).
Dawn of War 2 is an ideal MP game; it just lacks the base building aspect of RTS games and instead focuses much more on tactical squad control and micro management of those units. There is also the hero team battle against waves of grunts mode (I forget its name) as part of that game.
Original Dawn of War is a more traditional RTS game with base building and expansion; I'd say Dark Crusade is the generally accepted most fun for your money title; Soul Storm is the latest one, but has some balance issues from what I've read (though it has the most number of armies of all

I played both Dark Crusade and DOW2, both a lot of fun. I got confused with DOW2 in terms of all the different releases and expansions I dont really know which one is the full package now.

thanks for sharing RON: Legends, looks really good.

Men of War and MoW: Assault Squad are both brilliant. And MoW can be got really cheap.
They are both really cheap at GetGamesGo at the moment, actually.

http://getgamesgo.com/product/men-of-war
http://getgamesgo.com/product/men-of-war-assault-squad

AI War: Fleet Command is a pretty good game.

http://www.arcengames.com/w/

Everything about this game is so wrong. In the best way possible. Thanks for sharing! I will have to setup a virtualmachine with Windows XP to make it work but at least found the game and the sountrack is amazing!

i like sins of a solar empire as well

turns up the steam sales may be profitable for me right now.