With a lower end system, you're going to want to go fairly minimal. Grab the key addons that change the look and feel and leave it at that. You can start adding bells and whistles once you have your stuff setup, but be prepared to drop the unneeded ones if they start affecting your system.
Unit Frames - I use Pitbull, but there's a plethora out there. AG Unit Frames, Perl Classic, XPerl are all good options. Try a few and find one you like.
Action Bars - Dominos or Bartender4 are pretty standard. Try changing the scale of individual bars and hiding unneeded ones when you don't need them.
Raid Warnings - BigWigs or Deadly Boss Mods, both do the same thing. I prefer BigWigs, but in essence, they are the same.
Threat Meter - Omen
Damage Meter - Recount is pretty much standard now a days. However, this can add quite a load on your resources, so I'd suggest not using it at first, and then adding it and configuring it to take up as little memory as possible. (Track only Boss Fights, possibly track only your damage, don't track out of group, etc)
After that, it's all bells and whistles as each addon will increase the amount of memory used.
A bigger part of what you're looking for is also proper setup. You can have only a few addons installed, but if not properly setup they will still clutter your screen. If you search around (or ask nicely), Theo has made some great posts about his own UI. It would be the epitome of "uncluttered" if it still remains how I remember it. I have also uncluttered a lot of
my UI, and while I do run with quite a few addons (that would make a low end system cry), the time I took to set it up made sure I could still see what I'm doing. (I still have some opportunities to unclutter more, but I do a lot of button swapping as I play, so I have several "storage" bars to make that easier).