|You will almost certainly have to put the computer in the hold, you wont be able to take it as cabin baggage. Because of this I would remove the hard disk and take it that into the cabin with you, it should fit nicely in your husbands jacket pocket.. That is the most sensitive part and the part that is most likely to get damaged in transit.|
Pack it well in a suit case and you should be fine. With something that big you might end up with excess baggage charges. Worth checking with the airline before you depart. They are the people to answer all these kinds of questions.
Bear in mind that the voltage in the UK and US are different. You will need to ensure the the PC can take the 110 v in the US and not the 240 volts it is used to.
With most power supplies this is not a problem but it is worth checking. They will have either a switch to change over or be auto sensing. If they are auto sensing the there will be a label that say 120/240 v 56/60 Hz.