I'm a big proponent of rolling your own for DAW's. One of the big reasons is that you can choose all of the individual components to make sure that everything is quality and fully compatible with your pro sound card - and that the OS is configured so everything is as streamlined as possible - so that performance is reliable and glitch free. Obviously the needs of a multitracking or virtual instrument machine are a lot more than just a mastering DAW - but if you actually want to be able to do some heavy lifting with your DAW roll your own is the only way to go as far as I'm concerned. The other reason is that by building a custom box you really learn what's going on with the guts of the thing making trouble shooting a lot easier to do than if you have to rely on an outsiders "tech support". Of course - OMMV.
As far as Win98/Me - I don't think this is a good choice for a DAW at all this OS does not handle threading priorities correctly - in fact it's not really a true 32bit OS but a 32bit shell over a 16bit OS (btw - those bits I'm referring deal with buss throughput and have nothing to do with audio bits).
XP has one major thing that disturbs me: the activation scheme. To me when I turn on my computer I want it to work - period. The idea that after I've swapped my C drive 3 tmes I have to make a phone call or go online for an activation code(and I think the best policy is to keep DAWs completely off of the net) just rubs me completely the wrong way. Even though this is a "tiny inconvenience" if I have alternatives - I'll use them. For this reason I've stuck with Win2k for all my DAW boxes.
For apps like Sonar, which depend on the Windows Kernel Mixer then you'll get better performance from XP - but for the app I use, SAWStudio, I did a test with a dual boot Win2k/XP system and found absolutely no improvements in it's performance whatsoever from using XP. I'm sure at some point there will be a reason for me to actually want to "upgrade" - it just hasn't happened yet -
Best regards,
Steve Berson