I think the advice here is pretty good.  Always difficult to tell on the PC front, but it does look as though that would be probably the most significant purchase (but then you can also use it for other stuff as well!). I got my rig from Computerplanet (good service and custom builds).
 
 Aside from that I would go for something like the Focusrite Scrarlet 2i4 (around GBP149 from most online places) as a pretty solid USB interface for the price.  That will give you 2 inputs which should be enough for what you need now.
 
If you are tracking and mixing using 'phones you should budget for something there.  I have a pair of AKG K702 which are superb, but because they are open backed you may find the noise bleed too much for vocal tracking.  Alternatively get some in-ear buds for tracking, but use the better phones for mixing.  If you go this route you won't need to spend large amounts on monitoring, which then needs room treatment and everything else.
 
Lastly, as Jonesey suggests do get a small midi keyboard controller. 25 key is enough to make your job so much easier.  A bunch of options here, and something like the Arturia KeyLab25 (at around £150) includes a whole bunch of analog synth patches with basic editing capability.  By all means check out Omnisphere but be careful.  I did, and immediately bought it (and the v2 upgrade as well!).

molsp
Ken, is that link meant to take me to a shopping cart with stuff in?

Yes, and sorry if it did not. It was a Novation Impulse 49, RME Babyface and a pc (the one in Jonsey's link).

thats definitely do-able for me, thanks Ken and Jonesy - So, along with my speakers and mic I don't need anything else??    
 
PS Jonesy - sorry to sound like a numpty but how would I use a midi keyboard?

molsp
 record directly into Sonar and listen to output via an old pair of speakers
(Edirol MA-20D) which are connected to the back of the fast track machine.
 
The problems/bugs I have:
 
* When I mix I have to rely entirely on headphones as my speakers don't give me stereo.

If you have two speakers why are you not getting stereo? Is one of them dead? 

No, they both work but if i pan something hard right i don't hear it - i feel its to do with how they are connected to my fast track. Each speaker has a jack plug and they go into a 2 to 1 jack plug and then into the fast track. 

Right, your Fast Track has separate outputs for Left & Right - replace your 2 into 1 with 2 separate cables
 
You will connect your Midi keyboard via a spares USB socket on your computer - we can help you get it configured inside Sonar

Thanks everyone for all your input - I think I need a new computer as well and I think that carillon is the way i will  go.
 

You don't need a new computer.  Your processor and 4 gigs of RAM should be fine unless you are doing huge track counts with lots of effects or VST's.  In my opinion SONAR needs a bit more RAM and CPU power than something like Reaper, but it should still work fine.
 
More important is the latency of your audio interface and your buffer settings. 

I disagree, for all of the reasons outlined above by myself and others

It's probably a bit of both. I will say that a high quality interface gets a lot more use out of an older computer.