SQL Server Disk Sub-system (overhaul) Performance Questions

I 'inherited' a group of SQL Server server class machines. They are
true server technology but the disk sub-systems are lacking. There is
one hot-swap backplane that all the drives share (with one SCSI
channel) thusly even though there are three logical drives (composed
from 6 to 8 hard drives), they all go through one channel. This is
creating a performance issue that is noticable and can be seen in
various performance counters that Microsoft recommended one should
monitor in terms of disk I/O. For a cheaper 'fix', I can add a
seperate two drive bay (with its own SCSI channel) with mirrored
drives. I would then mostly likely place the transaction log files on
this new channel. Or I could place the indices filegroup files on
this new channel for DBs with mainly searching going on (not much
updating). If I went this route I would be leaning towards the
transaction log move since the second method would require me moving
DBs around quite a bit. Any input on this solution (besides spending
more money)?

What I would prefer to do is get a better server class machine or add
an external drive bay solution (not a SAN). I would try to get three
or four SCSI channels in the new hardware to split the different
file/filegroups out (i.e. transaction logs files, data filegroup,
indices filegroup, etc.). My only concern here is: would this more
expensive solution be worth the money? As far as replacing servers, I
have only two kinds of experience...replacing somewhat underpowered
servers with slightly less underpowered servers and replacing overkill
servers with even more overkill servers. In both cases, the disk
sub-systems were fairly equivalent from the old system to the new one.

Will going the three/four channel route really get data moving along?
We have one server in particular that hosts a database (one of many on
it) for a web application that gets decent traffic (it is a private
login based system for internal use and external use by our clients'
agents). Periodically throughout the day, there are 2-5 minute bursts
where performance slows to a crawl. I want to spend more time
profiling queries and such before recommending we spend more money,
but the folks I am working for want quick results and there is quite a
bit of stored procedure logic to profile and investigate. I know the
disk sub-system is definately in need of an overhaul, but I would like
to get an idea of peformance gains from adding either one additional
channel over the existing single channel as well as going the
three/four channel route over the existing single channel setup.

Any information would be greatly appreciated.

Regards,
Tony