having two sets that are rated same speeds and timings isn't a bad idea at all. Its having two sets of ram at completely diff speeds and timings that is bad, but as long as u can set a config that satisfies both sets ur fine. thats y i ended up using the patriot 1200mhz ram in a clients computer and traded em for a cheaper set of gskills b4 i sold my rig.
the overclock didnt really have a big hit in performance, but i did notice it load quicker, dont forget ram doesnt really have an impact on performance as long as u have at least 2gb ddr800. ddr3 is a diff story...