I've seen your desktop and all the qwerks you have there. So my advice to you would be this.
Buy a Tower On its own.
This one has win 7 installed ready to go. http://m.ebuyer.com/662319
That's what you can have New without any problems, you plug it in, self installs windows and away you go.
Ok why have I chosen a new tower for you and not the upgrade. Your existing pc is getting older and who knows when it's going to throw its hand in.
You can't always run Windows xp programs backwards with Windows 7, as there were lots of printer problems through windows 7 as xp. The program your using are as old as win xp its self.
Now with two towers you can slowly copy and try to run your programs on the new machine. As you work out what is compatible and not.
There are a few roads to go down to getting your new computer to work with your new tower but its not always plain sailing to get them working.
Because you have your original tower you can just plug that in and get on with it when a program no longer works over on win 7.
The link I gave you above is showing you what new can buy with win 7 pro installed on it too. Take off £100 and you can have it without windows 7.
But its new and reliability speed is part of it too. Modern computers just work now Chris no arsin around.
You can run Windows xp inside win7 pro and above.
We all know Microsoft ballsed up win 8 so if windows 10 gets positive feedback your entitled to a free upgrade within the first year of windows 10 ( There was no 9).
I will try and call you in an hour to explain it better. Holding onto old hardware will only cause problems.