to speed up map loading times, turn OFF texture compression (from the menu). only advised if you have a 64mb graphics card.
dunno about SP but this works very very well for MP, i am now first into about 90% of games, its not unknown for me to capture the forward depolyment area in the sub level when several people are still loading the map.
other things to do are adjust the memory allocated to RtCW, see the readme.txt (IMPORTANT!) that comes with the game, but note, what is doesnt say is any settings you apply to SP do not take effect in MP, you have to do it there too. RtCW allocates itself 72mb in SP and a pathetic 52mb in MP, if you have 256mb up it to maybe 128mb for BOTH, certainly you should change MP to use at least 72mb.
"without taking the detail down"
- dont be daft, take down the detail for MP, even I do this and my system pretty damn good (see link in sig). you dont have to do it a lot, just a couple of things. 3 things you wont even notice, that will improve performance and/or conenction, is hit ~ (actually, the key in top left of keyb, under escape, ¬ for UK kerboards), for the command prompt and type:
/cg_brasstime 0 -this just tells it not to bother drawing the empty bullet casings.
/cg_shadows 0 -takes away shadows, noticable performance increase and shadows dont help you anyway.
/s_musicvolume 0 -turns OFF (as opposed to right down) music, noticable performance increase. you dont get music in MP anyway except when looking at servers and at end of map.