1. On my rig (i've got a 2.66ghz quad core, a Geforce 9800GTX+ and 6 gigs of RAM), i couldn't get even the enhanced shaders to work. I guess it's a mid range PC, so it might give you some idea what to expect. That mod can be toggled on and off from inside the game, so you should just try it and see if the performance hit's worth it.
2.
CONELRAD's pretty decent as well. You can always combine them.
3.As far as color mods go, i like
Fellout NV a lot better than UWRL. Might be worth installing that. The
weapons retexture project has a minimum performance hit and looks great.
New Vegas Restoration and
Freeside Restoration are two other great mods that make Vegas look a little better and closer to what i think Obsidian envisioned. NVR is somewhat performance heavy, but i find the change is worth it. I'm also gonna try
Dog City Denver. It's a restoration of a city that was in the original Van Buren. Haven't tried it yet, but it shows lots of promise.