i tried using some older left over GPUs, couldn't really get any of them to work. older GPUs do not support the specific instruction sets of the current generation of GPUs. anything older than 4-5 years old, proably won't work.
if you want to try mining, get 1 or 2 Nvidia 750ti you can use them...
i don't really do any settings for X11. just, depending on what miner you have:
./sgminer --kernel darkcoin -o stratum+tcp://mining.darkcointalk.org:3333 -u Weblogin.WorkerName -p WorkerPassword
and you can always tweak your core and mem clks.
i
i did not notice a drop in hashrate, but not sure i got a good A/B comparison, since i am constantly tweaking my settings and which coins i'm mining. next time to flash a GPU, i'll make sure and check.