3000 C100 0761-3GG Processor Upgrade Questions

[Cal Culater]

This machine has its original Celeron 370 1.5Ghz processor and I would like to upgrade it with reasonable bang for the least bucks.

I understand that a Pentium M 760, 750, 740 can be swapped straight into the slot.
I will probably go for the 750 1.83GHz as they are very cheap (1/3 the price of the 760) on EBay.
Am I correct in thinking that there will be far more benefit from going from a Celeron with 1MB cache to a Pentium M with 2MB cache than going the extra mile for a 760 over a 750?
Are there any heat issues using a Pentium M instead of a Celeron? Will the board/bios already know to take advantage of Speedstep with the M processor?

We have had the machine since new and it has all the original bios, drivers, etc. (I am a great believer in "if it ain't broke, don't fix it" so have never updated any of these).
Will a CPU swap to a Pentium M "just work" or are upgrades to the bios or drivers likely to be required?

The machine has 2 x 512 MB of PC2-4200 RAM at present. I use XPSP3 but am considering going to Windows 7 later.
Is there likely to be a worthwhile additional benefit from going to PC2-5300 over 4200 in comparison with doing the processor upgrade?
If I was to buy 1GB of PC2-5300 and install it with an existing 512 MB of PC2-4200 won't it all run at 4200 anyway? In which case is 1GB of 5300 better than 1.5GB of 4200?

Cheers in anticipation, great forum BTW