Dell doesn't seem to have them on their web site anymore.  

I need to replace one of the ones I currently have, and then add a new one.

I use TrueImage to image the windows XP config, so it will only image
back to the same hardware. This is like Ghost.

I don't want to have to redo my image by starting installing apps and
configuring settings from scratch.

thanx in advnce,

sherpaj,

As was mentioned in the comment below, if the model is not offered on
the website, you cannot purchase a new Dimension 4550 from Dell. 
However, it is still possible to purchase an almost-as-good-as-new
refurbished Dimension 4550 from Dell Outlet.  According to this search
on the Dell site:

http://outlet.us.dell.com/Dispatcher?target=InventoryPage&action=filter&lob=DIM&unique=1080659583899&sessionID=ApO9OsST!1045422130!-1217671166!1080659581727&tgtSeg=I

As of this writing, there are 40 refurbished Dimension 4550 computers
still available.

The definition of a "refurbished" computer is supplied from their
website http://www1.us.dell.com/content/topics/segtopic.aspx/reliability?c=us&cs=22&l=en&s=dfh&~ck=mn

"All of Dell's new computers come with a 30-day total satisfaction
policy. As a result of this policy, some computers are returned. While
only a small fraction of this equipment is returned because of
technical issues all systems are put through the production process
again. They are taken apart and rebuilt to original factory
specifications, then retested to ensure the same quality and award
winning workmanship standards as new. We even use brand new boxes for
packaging!

Because our systems are refurbished and then retested to meet Dell's
high quality standards, we offer the same limited warranties1 as the
new systems.

Dell has achieved the industries highest awards for customer service
and customer support, and we include the very same service and support
levels with each Dell Outlet system we sell. Additionally, if you are
an existing customer for any new Dell system purchase, your customer
service and technical support remains the same.

With every Dell Outlet computer you purchase, you receive the same
award-winning technical support that comes with each new Dell system -
24 hours a day, 7 days a week."

With this information, you should be able to get two quality Dimension
4550's that will meet your needs!

Sites referenced
----------------
Dell Outlet
http://www.delloutlet.com

sherpaj-ga:

If Dell doesn't have this model on their website any more, then it
will be almost impossible to buy two new units as these were made to
order.

You can, however, buy off-lease units from Dell Financial Services'
online store. Currently, they only have one Dimension 4550 available:

http://www.dfsdirectsales.com/dfsdirect/desktop_inventory.asp?category_id=11949&family=Dimension&model=(NO%20O/S)%204550

aht-ga
Google Answers Researcher