It may depend on what type of genre you like.

I'd suggest the AGEOD's recent games Birth Of America and AGEOD's American Civil War are very good strategy games that don't tax your graphics card too much. They can use quite a bit of CPU when processing turns (but that is variable by the player too). These games very good strategic games.

With regard to Take Command 2nd Manassas I'd say that it can be a bit graphics and RAM heavy if you want to play at high resolution however, you can chose to use lower resolution and as the units are 2D sprites on a 3D map it is about as taxing on graphics cards as RTW I'd say. However, it is a tactical game, not a strategic game, if you understand that it is more like a Civil war battlefield simulation. You are making tactical decisions, no strategic planning is involved at all.

I'd also suggest the Paradox games mentioned EU3 and Hearts of Iron, two rather similar games in terms of gameplay.

I've heard good things about Galactic Civilizations. From what I've seen about it online it also have a very good AI and seems to have good gameplay.

May I suggest the following strategy wargamer sites.

http://jaguarusf.blogspot.com/

http://www.wargamer.com

Alternatively go visit the wargame developers websites.

Cheers,