I do most of my artwork on a tablet actually including the rough sketching. The glass screen effect I get around by using a anti-glare screen protector which adds a layer of finely textured silicon/plastic stuff and if thats still not enough you can switch to Wacom felt nibs which will increase the friction on the screen dramatically. Some machines such as the Lenovo X60 and X61s have screens that have already been etched to give a similar textured feel. I agree that RAM needs to be maxed in order to have a smooth sketching experience for the most part. I've reformatted a HP TC4200 with an external USB drive without any problems so it can work. I do know that only certain drives can be used but there are some other than the manufacturer options which are cheaper (mine was sub-$100).
The primary advantage of working digitally is layers, undo, scale/rotate tools, and cut/paste. As well as less clutter and ability to carry your entire art production studio in a backpack. I have been working in vista for the last 4 months and while there have been some problems (mostly hardware driver related), I have got it all sorted out at this point and everything is working fairly well. I have a co worker though who is having a lot of problems and wants to revert to XP as well. XP is safer as its tried and true. I think if you have a decent amount of technical knowledge and willingness to tinker you can get a vista box set up working well but its not worth the hassle for many.
Check out my tablet pc made artwork at www.JLL-design.com