q1. which is the best combo? - The 6300 and 760 would probably game a little better in most titles thanks to the fact most games are not fully threaded
q2. height of ram an issue? - it can be; the most popular budget cpu cooler (hyper evo 212) won't have an issue with taller ram, as you can clip the fan onto the sink higher up the sink to get above the tall ram. Obviously this isn't an option with all cpu coolers, so i suggest you do your research carefully to insure the cpu cooler will fit. obviously all closed loop water systems will work.
q3. can i overclock on the extreme3? - to 4ghz, yes. heck you might be able to get up to 4.4 as many piledriver cpus can get that far on stock voltage. That said you will not be able to get much further. I would not use the 8 core fx8320 on it... it's just not a good motherboard. Nor would i use that board for more then overclocking on stock voltage. A word of warning though. generally you don't want to say the words "overclock" and "piledriver" together with any motherboard sporting less then a high quality 4+2 (for an asus board) or 6+2 (for everyone else) vrm design with vrm heatsinks. That asrock has no heatsinks and is a 4+1 board... it will not run a 8 core fx... and you run the risk of killing the board and starting a fire if you push the overclock too hard on a fx 6300.
well... the best choice in your price-range would probably be the ASUS M5A97 R2.0; unfortunately it's sold out... not sure bro... no real good options in your pricerange on that site. I see the extreme3 is the r2 version, which is a bit better then the one i was thinking about. I would just stick with the asrock, you should be alright... just don't do any FSB/CPU frequency overclocking, stick to the multiplier... and you should be alright for a nice moderate overclock on that fx6300