From Query Analyzer I attempted to run an update to a 13GB unindexed table (about 20 million rows) in a database sized at 30GB containing only the table in question and a 400-row joining table. After 36 hours running I cancelled the query. Here's the query code:

UPDATE P
SET P.Sys = W.Sys
FROM PAC P INNER JOIN WORK W ON (P.CID = W.CID)
WHERE P.Sys IS NULL

Specifics:
WIN2K Standard Server
SQL 2K Standard
4GB memory
Promise ATA RAID 10
Dual Pentium 1.8 processors


PerfMon samples:
Processor: between 1 and 3% utilized
Avg. Disk sec/transfer: between .008 and .800
Avg. Disk queue length: between 1 and 140
Avg. Disk sec/write: between .005 and .800
Avg. Disk sec/read: between .007 and 1.500

By these statistics I infer that my disk subsystem is woefully inadequate, but I'm also wondering if memory could be a problem since memory usage is at about 1.9GB during the attempted UPDATE.
I'm pretty green around the ears when it comes to tables of this size, but a very similar update query ran successfully in about 40 minutes when the table (despite the lack of indexing) was about 1GB smaller.

Any ideas about how I can resolve this problem without spending a ton of money on a big box, SQL Enterprise and 64GB of memory? Will a SCSI subsystem be enough? If so, should I use fiber or would Ultra160 be enough?