At this point in time, I would suggest you upgrade rather than replace *IF* the result of upgrading will satisfy your needs for say the rest of this year - maybe even do both upgrades?
You allways loose *loads* when selling computers that arnt still curernt geenration, compared to the day to day value of having the thing.
intel mac support is only just getting going - I dont know if drivers have to be platform specific or not - ie whether you need specific intel mac drivers - I would assume so.
On the PC side - theres another thread
here the details my recent experience with WinX64 - the summary is I had to go back to WinXP due to lack of available X64 bit drivers for some of my audio gear.