Is there a performance difference? Yes, there probably is. Is it noticable? Probably not even in the slightest except under EXTREME load, and even there the performance difference will be negligable.
I'd choose the cheaper option - top end processors do not provide value for money. High end hardware is simply a cash cow for manufactureres - the hardware you pay top dollar for today you can buy for half the price in less than six months, as newer products push them down the performance scale (and thus make them less desirable to performance freaks).
About your aside - Intel certainly has plans for a new 'socket' for their chips - this *may* be worth hanging out for. I'm generally not a fan of hanging out for new tech - you'll be waiting forever. First you'll want a mobo with the new socket, then you'll consider maybe you want to wait longer and get that mobo that supports PCI-Express, then you read about a new Northbridge chip that will improve performance further, etc etc. The only exception to this for me is a truly 'ground breaking' technology - and they are usually expensive, and often a bit flaky in their first implementation (or at least very much more impressive a few months further down the line). Of course if you buy your new motherboard now you will cut yourself out of the new processors upgrade path - but expect to see P4s in their current pinned configuration (and at faster speeds) for a while yet.
Basically, unless you're cashed up, and a big fan of the bleeding edge, it's very rarely worth waiting, and rarely worth paying top dollar for.
