Find out your firmware version by pressing :
right * left left * left *
Go to the bottom and note the CDA number.
Go to http://www.seuniverse.com/step1.php and follow the instructions there. Use the CDA string from your phone in the CDA dialog of the application.
You should use Europe 2 as the region.
Apart from that, upgrade is seamless. No hassle, walkman feature is tres nice.
On O2, on the contract I chose, the K750i is 70 euro and the w800i 229 euro. A 1GB memory stick duo cost me 43 euro on ebay, a 3.5mm converter cost me 5 euro off ebay and the upgrade software was 10euro. So I get a w800i equivalent with extra 512mb memory for 128 smackaroos. Around 100 quid saving ! Don't buy a w800i ! You can even get a w800i fascia off ebay that will fit the k750i for around 20 euro, if you like the look that much.