I currently use pipecall from pipemedia, it costs 2.9 a month comes with a 0845 number, I use my stereo speakers and a pc mic to make and receive callls.. The service is such good quality I have dropped my landlien. Admittedly you can tell your voice has been heavilly encoded, but for a noraml telephone conversation its great, and the fact my voicemails and faxes goto my email address, thus following me around is great.. VoIP is really becoming a reality now! id say givie it a go, you only sign up for one month at a time so give it a trial..
www.pipemedia.com
ps the software i use is the x-ten soft phone. but you can buy a SIP ready handset for £60
oh the only thing to remeber if your uploading files on bit torrent or to a server, you might gfet some delays in your voice call.. (need QoS)...