to check on other parts of the PC that makes up the "speed" is a little hard to decide :
hard disk speed = regardless of what storage what buffer, if speed is important might as well get a SCSI 10'000 rpm hdd ? but then again.. the cost is big time.. or maybe use 2 hard disk to RAID for the speed. I don't know. but it's better to use Serial ATA hard disk compared to Parallel ATA. (according to my experience findings, SATA hard disk generate less heat too! and HEAT = slowdown)
RAM = so what if we use the extra money to focus on ram? windows xp pro supports a maximum of 4gig ram, and get the highest ram speed possible. and spend a little extra to cover the ram with aluminium/copper cooler master heat sinks of some sort to boost "invisible difference". but probably makes your ram last longer.
CPU = AMD/Intel, either one is your own individual preference between chopsticks vs fork and spoon. (i'm a chopstick guy. teochew.)
Soundcard = latency etc etc etc. which one's right nowadays? everything claims itself to be low latency.find out yourself.
lastly, for those of you lazy bums on the computer who don't know how to maintain, start learning :
OS = defrag often, don't install nonsense. set your paging file memory right. use a disk-image capture software for fresh configurations. set your DAW settings right on the buffer etc.
Miscellaneous = if you're using a PC DAW to record, besides just focusing on the above speed thingy. do focus on PC stability too , core2duo's need high power, don't use your $70 stock casing with 400watts. usually play safe most C2D cpus need 450 watts, if not 400 watts and above + Truepower. meaning buying seperate power supplies from companies like cooler master / antec. and get a good mainboard because if you subject constant abuse to it (doing recording), you're going to blow the capacitors sooner or later. make sure ventilation is there. try not to smoke infront of the PC (believe me, you can tell 80% when a PC owner is a smoker by opening up the PC)
hope these tips help.
Cheez : at first i thought the same as you regarding HD size matter, but now my PC's running 250+320+320+320 gig hard disk, actually it's fine. probably the reason why it'll run slow is due to bad habits. when you get a huge @ss hdd space, you'll tend to save..and save and save. and almost lazy to clear / defrag. hahah.
70¢ worth