The 5600+ X2 is still cheaper then the E6300 and the 5600+ beats the E6300 by even more in benchmarks then the 5000+. A 5600+ fully overclocked to highest stable and an E6300 with the same cooler overclocked to highest stable should be fairly even.
I am afraid you are a bit out there, I am an AMD fanboi but clock for clock the C2D (any C2D) is 15-20% faster than any AMD offering. So, as I have said before, at stock speeds there are a few AMD chips that can beat C2D in many benches, mainly because they are clocked higher at stock, if you want a real comparison find a review of the Opty 165 at stock versus the E6300 that are close at stock speeds, I cant find the one I read that was from the week the E6300 was released but in all benches the 6300 was averaging 23% faster (slightly higher stock speeds), both had 2MB L2 cache. Once you decide however to overclock any AMD or C2D to near its limits generally you are talking a 30-40% speed difference.
As I again mentioned before, look on these forums at the SuperPI 1M thread, the fastest AMD I have seen hits around 25Secs (I think it might be my 4000+ @ 3.27Gig, or it was at one time), the fastest C2D I have seen is slightly under 14 seconds, by my reckoning thats around 43% quicker.
For the record, 1st Gen Windsors with 2 x 1MB L2 Cache were the high end chips in the 4400, 4800 and Opty x2's (although in some they appeared as Denmarks, same chip really), Windsors have moved on to 2nd Gen with 2x 512KB L2 (now supporting the low powered 90Nm process), although they are still making some 2 x 1MB Caches for 5600 and 6000, Manchesters were the slightly lower end 3800, 4200, 4600 with 2x 512MB, recently the 65nm Brisbanes added to that, again with 512KB x2 L2 cache with the half multiplier (Apart from the 5000 @ 2.6 confusingly) as there is also a low powered Windsor at 2.6Gig.
AMD is only manufacturing Brisbanes and Windsors currently (as far as I am aware)in their desktop market sector, all with 2 x 512KB L2, the top of the range 6000 is a Windsor core I beleive, I may have the odd thing in there wrong (dont think so tho) but generally thats the score.