I'm really keen on getting hold of the new Hercules Prophet 9800 Pro graphics card, and have found I can order it online from the Guillemot site in the US for what is a relatively competitve price. They will ship it to me in the UK, but the site warns that, in addition to FedEx costs, "shipping charges do not include any Brokerage Fees, Customs Fees or Taxes that you may be charged."
Does anyone know if I'm likely to incur extra expense before I can receive the item?
It is very rare for self imports from the USA to be any cheaper than the UK for the reasons you are already aware of. The best thing to do is find someone who is going to the States and get them to pick the item up for you. There are many thousands of small computer shops who sell at very good prices.
I used to order Toys from the States and if they are above a certain value (which is quite low £20 or something) then you can get stung for 17.5% vat and on top of this there are custom fees etc and you end up paying something like 23% on the cost of an item.
I suppose it really depends on how much you save on the item you are buying, remember though, how easy will it be if you need to return it for repair?
H.M. customs limit for tax free import of goods is £18. So to get anything tax free it needs to be £17-99 or under and this also must take into account any carriage and postal charges. Anything over £17-99 (goods + post/carriage/packing) and you will have to pay VAT @ 17.5%, possible duty which depends on the type of goods imported and also possibly an administrative charge for the postoffice which is currently around £3-85 per package. The Post Office are allowed to charge this by international agreement. Its a standard charge for collecting the VAT etc. on behalf of customs. You may also have to go your local sorting office to collect the goods. A very few packages do escape the net but you can't count on this. There are different rules for the Eurozone - depends whether VAT has been charged in the Country of origin.
There is a huge department of HMC&E devoted to this and examining packages for porn. As the porn laws have relaxed slightly recently, more attention is being given to stinging people for tax and duty on innocent goods.