I f you want to get into using a PC for making music a good idea is to try out a few progam demos and find out which one suits you. Go into tower or Easons and look at the demo disc which comes with magazines like 'Future music' or 'sound on sound' or 'Computer music' . Usually one of these has the latest demo versions of various recording and sequencing software. YOU DO NOT NEED TO SPLASH OUT LOADs OF MONEY (thats enough capitals) on an expensive soundcard at this stage. Your bog standard PC sound card will run these even though recording audio may have a bit of time lag between the live and monitored signals (the Latency).
From what you have said 'ABELTON live 4' would suit your needs the best. This program breaks down a song into blocks and makes it very easy to add pre-arranged drum patterns and other stuff on the fly. Its a modular program and has a very good demonstration program in the demo which allows to you to build up a song and get a feel for the process. It has alot of sounds and also allows you to record audio in chunks eg. verse guitar/ chourus vocal..
I use Sonar 4 and reason. If you are going to get into arranging 'Reason' is truly the dogs b****x . You cannot record into it, but you can sequence everything - drums,bass, synths and import your own samples. You can then export the arrangement and sing over it in a recording program like cubase or the others. You can also wire reason into these programs if you like.
If you are going to get a soundcard - perhaps get an external usb on with 2 audio inputs and a midi in/out. USB devices are very easy to set up and usually have 'control panel' within windows which is easy to access and set up your configuration.
Edirol is a good make or M-audio. You can get these much cheaper on the net than in Ireland .
http://www.netzmarkt.de/thomann/thoiw6_artikel-173439.html
Here is an example of a 2 in audio usb and midi interface for 155 euro! (23 squid shipping)
Even cheaper maybe from digital village in England.
Hope this was helpful