Interesting questions.
I think the (correct, IMHO) conclusion you reached about moving from your 2.53/533 to a 2.4/800 applies to the motherboards, too. Right now there is a measurable benchmark difference between the 845PE and a new 875, just like there might be between the two procs. But that difference is going to be irrelevant by this time next year. It will matter even less three years from now when you might be ready to buy a P5/5.0. (They will be out sooner than that, but I don't think they will be a midrange product until late 2005.)
The relative performance difference between these boards won't matter over three years of ownership. So you should get a good, cheap 845PE board, right? Maybe not, because time has the same effect on price that it does on performance. At the extremes of your chipset example are the Abit BH7 (845PE) and the Asus P4C800 (875). There's $210US of real estate between their pricetags. Over the three years I think it will take for five gig procs to fall into the affordable category the difference comes down to twenty cents a day.
Bottom line: Nothing matters and in the end we're all dead so who cares anyway?
I love the 845 chipset but I would lean towards the board that gave me the most options going forward. Procs in the 3.5-4.0GHz range are going to become affordable well before the 5.0 and would represent a substantial boost over your 2.53. If you want to make that jump having a default 800MHz FSB would be nice to have. Ditto for D-DDR capability, native S-ATA drive support and AGP8X. An 845 board can't do that.
I would wait to see what the 865PE boards look like before I bought an 845PE. The pricing looks to be comparable, at least after the initial gouging period, and I think its going to give you the best bang for your buck.
Also, know that you can wait for the prices to fall and the BIOS' to be debugged without missing a lot of performance. I agree with you that the 845G isn't a board I'd want to live with for the next three years, but its good enough to do the job for the next three months.
Hope this helped-
BHD