I've had a 3570k at 4.5 ghz and have a 3770k at 4.5 ghz and a FX 6300 that will clock 4.4 ghz (on a crappy zalman 9500 so you may get your 5 ghz with the closed loop water cooler)
I honestly can't tell a difference in anything but benchmarks. The FX 6300 @ 4.4 and the 5870 and Adata SP900 128 gb ssd runs very well for the price. However in all honestly your 2500K will be faster than the FX 6300
If your friend offered you a significant amount of $$ then consider it, but over all the 2500K is the better cpu
I built the FX 6300 + 990FXa-UD3 for $203 shipped and got the memory with a $25 mir for around $57, basically the cost of a new i5 + cheap mb I got a FX 6300 + 990 FX mb + 8 gigs of ram so I went that route. Plus I hadn't built AMD in a few years.
So without rambling. FX6300 is a great chip but the 2500K is better. There is a thread of FX 6300 @ 4.8 vs 2500k (or was it 3570k) which showed the 6300 did ok but the i5 was better.
http://www.overclock.net/t/1421579/fx-6300-vs-core-i5-benches/100_100#post_20723992Edited by CravinR1 - 9/13/13 at 5:00pm