Until last March I played in the MA's on:
Dell Dimension XPS-T600
PIII 600
512 mb PC100 SRAM
32 mb G-Force MX-440SE
Soundblaster Live! Value
Windows 98SE
I'd turned ground detail to low and max visibility to 3 miles with 512 textures and no custom skins. My FPS were normally in the 15-25 range but would drop under 10 around many buff formations or in low lying fog. They'd run 35 FPS in blue sky with one or two cons around. Fires, smoke, etc. didn't seem to hurt my frame rates.
Some people will tell you that those frame rates are unplayable but it seemed smooth to me since I had no other point of reference.
First make sure your machine is clean:
*Scan your computer for viruses, spyware, adware, etc.
*Remove any unused applications.
*Turn off any un-needed background applications (auto-updaters, etc.) in Task Manager or Services depending on your version of Windows.
*Clear out your Internet browser cache, recycling bin, downloaded program files, Windows Temp folder, etc. using Disc Cleanup (if you've changed the default location for your browser cache you may have to clear it through your browser. You can also manually delete anything in the Windows Temp folder).
*Run Scandisk to make sure you don't have file problems or bad drive sectors.
*Run Disc Defragmenter.
*Before you log into the game turn off any anti-spyware, anti-adware, anti-virus and firewall programs.
Now log into the MA's and have fun. If you're still having trouble run 256 textures.
You may still find situations you'll want to avoid that may over-tax your system but you'll quickly learn what they are.
If that old dinosaur computer listed above can play in the mains any computer can do it.