Lol. i aint gonna get into a self-confidence speech, I personally enjoyed building my new PC cos of the acheivement side of it but if it's not your cup of tea then dont worry!
As for the pre-built PC I think it may be a bit pricey although i have never owned a pre built machine so I cant honestly say. I have also never heard of this TARGA brand so I may be wary of buying it.
1) Go out and buy the latest copy of PC PRO, it'll cost you £3.50 and it has reviews on some £699, £999 and £1500 PC's in there as a Xmas special, they are mail-order and they will tell you truthfully what they are like. (try other mags too, i said PC PRO as i have it here but there are others). £699 plus VAT works out at £820 so they will be in your price range, you can get fancy TFT's graphics cards and all-sorts of bells and whistles included.
2) Try and find a local PC builiding shop. I have a couple here in Preston, you can custom build your own PC with all the parts YOU want and they will build it for a relatively small labour charge.
3) Do some Google.co.uk searches for custom PC's, pre-built PC's etc etc and just do some research. I'm sure with a little bit of research you will find a better deal than what you have got there.
Please note also that i think that PC you listed doesn't come with a monitor? So that will be extra, it may not include VAT either as i dont think it says.