I use 512mb ati 4850 and tested all the mods I use for fps impact as I only get about 35fps and dont want to lose any. I can say none of the mods gave any drop in fps for me, this includes all HD skins (4096 for player and 2048 for AI) the truelight and true weather mod.
Truelight/Trueweather
This is a big mod by the guys from rddev mod group. A lot of work went into this one and at one point there were constant updates as it was still work in progress. They finally released this finish pack which works really well on all tracks. The mod tweaks various xml files for global settings and some specific to each track.
The truelight mod gives the game a much sharper and cleaner look but does cause some problems with shadows because off all the correct sun positions they put in. These are mostly noticable when you drive past big grandstands. However I prefer this look to the original look provided by cm.
The truewether mod gives the cars an insane amount of spray in heavy rain conditions. Again I like this compared to the original which feels really gamey, as if it was kept light to make it easy for players. The downside to this is it does make it harder to drive and see where you are going.
Enhanced audio version 1
Another mod that had a lot of work go into it, with a few version before this final one. The car sounds a lot more mechanical with this mods, you can hear the suspension a lot better and a few more screech and slip sounds. Also the ai sounds sound better overall. This mod works by tweaking a lot of xml like files.
HD car skins
Too many to mention but they look good and dont cause fps drop so worth downloading. Only problem is chosing which ones you want to use.
Artgear 3 +daftboy helmets
If you like having various good looking helmets and nice looking track selection menu than you need to try this mod.
Tyre mods
3 good tyre mods to pick from. Pirelli, rddev hd, or Dalhil v0.91. I like dalhil 0.91 as it's has the best damage imo.
Official pitgirl outfits
This is one of my mods. It does what it says, makes the pit girls have the outfits from real life.
Camera mods
Havent looked much into these but they are out there, giving the player all sorts of extra head movements.
Mods to lookout for
These are the mods that are in development and worth getting when released.
AIslow corner fix A massive mod trying to fix the cornering speed of ai on all tracks. It looks like they will include what I call the "george wiseman fuel fix" and the "DI qualiying lap times fix".
RDdev cinematography mod This will make all replay camera angles look like tv camera angles.