What do you mean by 'fast'? Will this be the ONLY HDD in the system? Can we assume Windoze XP (Pro)?
Considering today's HDD prices and what comes in a new PC, having multiple HDDs will really take advantage of your PC's actual prowess.
HDDs today are as fast as they can already be, and for many years now. Still, they're the slowest 'important' device in your system due to mechanical limitations. The theoretical max a HDD can put out is 85MB/sec. Know that this is so much lower than what the ATA-100 bus can handle. So, you have SATA-I (150MB/sec) or prolly SATA-II (300MB/sec) with the new PC. I consider them the 'holliest of all crap' in the industry (well, except the RIAA, of course!). What is the point of advertizing such speeds when the actual device cant even reach a quarter of it in the lab
Because I see no real difference in speed
for a single HDD implementation, I wld actually look at other factors such as:
1. Reliability: Diamondmax 10 design life is only 5 yrs vs Seagate's 5-yr warrranty. Shockproofing features?
2. Heat: translated into fan speeds, translated into fan noise
3. Acoustics: I dont want to hear it above my normal fan noise already!
4. SATA NCQ capabilities and how it affects performance when turned on/off
5. S.M.A.R.T capabilities. I'll want to know when it's time to change my drive before it really dies on me
6. Available utitilities fr manufacturer to assist drive formatting/partitioning/etc, run diagnostics and query general status
For all these, you'll need to look up the specs from the manufacturer's site.
If you want to see a head to head comparision: http://techreport.com/reviews/2005q1/maxtor-diamondmax10/index.x?pg=1
(Altho this is for the 300GB model, only its capacity differs. Only look at Diamondmax 10 & Raptor 360)
For me, I'd advise friends to start with a 2 HDD system from day one. You might want to look at XP Pro's RAID-0 Dynamic Disk feature which will bring out proper throughput for today's high powered CPU PCs.