Umm… buffer underruns are essentially a thing of the past for newer systems and drives due more (a lot more) to the the “Burn Proof”, “Burn Safe”, etc buffer/ cache management technology now onboard most decent drives vs increasing PC horsepower. You can still easily get a coaster with a high powered PC if you are trying to multi-task anything that produces a noticeable system load on the CPU. With Burn Proof you’d practically have to shoot the PC with a 44 magnum to get a coaster.
If you wanna get technical you can count the cache in the cd-rom as RAM 
I had completely forgotten about that part, but your right. From 4x I think cd-roms had a cache of about 512kb and with 8x they went as high as 2mb and are probably incredibly high with the 40x burners that are on the market now.
Nope…my 40x burner has a 2MB cache…but it can’t buffer underrun. If the machine can’t supply the data fast enough (which doesn’t happen on my machine), it’ll just slow down, catch up, and continue burning where it left off. Pretty much every new burner has this technology now, so there’s really no reason to have a huge buffer anymore.
Also, the media DOES matter a ton as to whether or not you can get the high burn speeds. Like I said earlier, my burner burns Fuji 24x perfectly at 40x, but can only burn Imation 16x discs at 24x. Higher than that will either produce a coaster or lock the machine. (The laser loses the guiding spiral and constantly searches for where it left off…a consequence of burn-proof) Solution: I only use good media.
Jman