Your PSU will cope fine with the new system as long as its of a good make and has little vdroop of the vcore (admittedly that has a lot to do with the motherboard) for a quad core Q6700 it will have to have a An 8-pin +12v EPS Power Connector, basically an 8 pin connector to power the chip , you can buy adapters but they are never quite as reliable as a hard wired one, but it will surfice, they are about £3 or £4 from Maplins if your PSU does not have one already.
Personally I would go for a mid range 650w-700w PSU like this one
http://www.ebuyer.com/product/124927 which is £45.
The system power draws a lot from a high spec graphics card so if you are using an oldish one then really 500w is ample if the old PSU is in good condition.
The Q6700 is a good chip with average bang for buck.
Asus make very good reliable motherboards and I would suggest you buy one and there is a myriad of motherboards out there with wonderful chip configurations!
Get a chip of P35 P45 X38 or X48 if you can.
Fir my home PC I use a P5Q3 Deluxe Wifi but thats £155 which is a bit out of the price bracket. I would recommend Asus as a good solid mobo and have had few problem with them compaired to the cheaper end of the market such as ASRock (avoid).
The price of Yorkfield processors has stayed about the same in the last year so I dont expect a price drop soon, but the i7 should start to drop prices soon but you will still be looking at nearer £500 for the same parts so thats a no-no.
memory is much of a muchness as lon as its a good speed rating, dont bother with less than 800mhz PC6400 but you will also gain very little by going 1066mhz PC8500 (making sure the mobo can take that anyway).
For £350 you can easily get the system you need and that will run circles round the Athlon system you have already.
Have you considered going AM3 Phenom II? I have not tried these yet but have headr good things about them, although I personally like the Intel chip and chipset.
