If you are prepared to pay out £100 for a chip and the phenom's won't work in your board then, I would be thinking more along the lines of getting some 2nd hand kit and clocking the hell out of it.
You can pick up Core2Duo's (C2D) very very cheaply for the lower range stuff, but they clock like hell usually.
I would personally suggest something like:
Motherboard: Either a cheap P35 or P45 chipset board.
CPU: something like e4300 / e5300 and clock the hell out of it.
Cooler: Cheap aftermarket cooler to support the overclocking safely. (Freezer 7 Pro for example)
Memory: Cheap 4gb's of DDR 6400 or above.
That could all be picked up if lucky for around £100 and be a MASSIVE leap in performance if overclocked to at least 3Ghz (which 'should' be very easy and pretty much guaranteed).
EDIT: However if you wish to keep it simple, then a phenom II chip would still be a massive improvement for you, but I would still definately overclock it to try and squeeze as much performance out of the system as possible, so also consider and aftermarket cooler, ESPECIALLY as your case is not exactly well ventilated.
EDIT 2: Just to back up myself here, in the members market (available once you get 250 posts.. but look at the forum rules for details.. you won't get access just yet).
Pentium e2180 (basically a slightly cut down C2D, but just fine).. is £20, and it runs at 3Ghz stably. That would leave you £80 for a motherboard and ram and a cooler.
EDIT 3: Not forgetting the 2nd hand phenom's can be picked up for well under £100 as well
