be sher what you are looking at, alot of computers can be upgraded to 1tb useing 4 250gb hds.

computers can naw that win me came out can be ugrated to amzing levals.
be for win me and 2000 the limet was basted on the amawndt of IRQ avlibal in the pc sytiom you wher useing. i for get what the limets orIRQ was but it was like 29 IRQs i think. an IRQ was used to list avibal slots for hard wher. the syitom otatily clams like 2-3 so it was not meny when win me/2000 it was asom.
 

alienware.com
Go beyond all limits with an Alienware ALX. With up to 4GB of high-speed RAM and up to 2.4 TB of hard drive space, the definition of "top-end" has just been officially changed.
-500GB (250GB x 2) Western Digital Caviar® SE 7,200 RPM w/16MB Cache [+59 or $2/mo.]
-800GB (400GB x 2) Hitachi Deskstar 7K400 7,200 RPM w/16MB Cache [+506 or $16/mo.]
+
Additional Storage Drive - 36.7GB Western Digital® Raptor™ 10,000 RPM w/8MB Cache [+133 or $4/mo.]

dell.com
-800GB Performance RAID 0 (2 x 400GB SATA HDDs) [add $535 or $16/month1]
+
-400GB Serial ATA Hard Drive (7200RPM) [add $257 or $8/month1]


so by this it seems the limet is a 400gb hd wich is probly slower.