I have to admit being a bit stumped here so perhaps someone else will help now the thread has been bumped.
I s'pose the first question i have to ask is what do you want to do with your system, as in what games do you play and want to play. What do you use your system for and how much time do you spend on it.
I honestly cannot see many on here recommending anything but starting again. I don't mean to disrespect your system but that Mobo you have is very dated now and i have had a job finding any relevent information on it apart from it being in many pre builds back in the day.
I have posted a link to a PSU below and a GFX card which will fit and play older games but you will struggle to raise the settings in any recent games.
Totals out at about a hundred quid. There in lies the problem as once you start spending that kind of money you may as well go all out and go PCI-E.
PCI-E components especially GFX's cards are nicely priced to encourage us all to switch over.
So depending on how much spare cash you have laying around will depend on what route you take. If you want to game at reasonable settings for some time to come then you will be very dissapointed with the GFX card i have linked to. If your not bothered about eye candy and are like most at this time of the year short of cash and you don't game much then a quick hundred on these two items and you'll be happy.
Now having said All that we still haven't established whether you actually need anything at all. I bet i could come round there trace the power button wires and jump start the thing in no time
So my suggestion is this get that side cover off, touch your case to expel any static from yourself and get brave. Trace those wires back to your power button, follow them down to your mobo. Then unplug them and use something to jump start it.
Whats the worst that can happen, well you could fry the mobo or blow the PSU but you would be No worse off than before.
Much more likely it will fire up sweet and then you will know it's just a power button which will cost you a few quid.
I know it's a long read but i really don't know what else to say, i have just tried to be as blatantly honest as i can.
Hopefully either my plan will work or you'll have plenty of spare cash and want a solid spec knocking up or at the very least one of the clever dudes will come and sort it out for us both.
Hope this helps.
PSU
http://www.overclockers.co.uk/showproduct.php?prodid=BG-007-AK
AGP GFX card
http://www.overclockers.co.uk/showproduct.php?prodid=GX-031-BG