As you say, Carrera have been through the wars [two insolvencies and three management teams] since you purchased the computer five years ago. They are now owned by the Watford Electronics Group.
Personally I would not consider a upgrade on a computer five years old, unless it it something very minor, as it would not be cost effective. New computers are available at some very realistic prices nowadays, and all will come with at least a twelve month warranty. Trying to fit old with new as in upgrade, doesn't always work, and I have found this out in trying to upgrade three computers over the past two/three years [money down the drain].
This past weekend I purchased a brand new [built to order] 2000+ 160Gb,518Mb,CDRW,Graphics card computer base unit for £167.32, which includes delivery for Friday and a three year RTB warranty.Try to better that price with a major upgrade in mind.