How is the E4500 or even 2160 better?
At stock, it's not. It's significantly worse, but you can overclock it to similar levels of performance as the E6750. Basically, all the Intel CPUs will overclock like mad, so the typical thought process is buy a cheap one and overclock the bits off it.
You originally didn't want to overclock, hence the whole debate about stock performance vs. discounted prices and bang for buck. The bang for buck on an E6750 is appallingly bad compared to a 21x0 series chip or even an E4x00 chip.
2180's will generally do 3.0-3.6GHz without a great deal of stress, 4300's are well known for running in excess of 3GHz, and E4500's are easier to get there because of the much higher multiplier. An E6750 on air might do 4GHz, but 3.6Ghz is more realistic, but the low multiplier and high initial FSB means you need a really good motherboard (not mATX) and RAM to get there.
What tatts is trying to say, is what Easyrider and many others have been saying throughout this thread - if you are going Intel - don't get the E6750 as it's poor value. Either go up one and get a Q6600, or go down one or two and get a budget CPU that will clock well and save you some money that you can either put towards a better graphics card or that 45nm CPU everyone seems obsessed with supporting.
Please accept my apologies if I'm sounding slightly narked. I'm actually slightly hung-over and that's not helping my mood

I do think you should just bite the bullet though. Whatever you do, you're almost certain to get a marked improvement on what you currently have, and that's the whole point, isn't it?
I'd much rather you came back in a week and said - "OK - you recommended this Easyrider, now help me clock it please".