the motherboard will come with a back plate (called an Input/output shield) to fit in the back of the case. it will fit fine as long as your case is atx or micro atx standard.
where it indicates 2 usb ports for the front it basically means it has a socket on the motherboard which can connect to a wire in the case for front usb ports, but if you don't have them then you just don't plug it into anything.
regarding 2.4Ghz processor, AMD processors basically do more instructions per cycle so they can run at lower frequencies than intel processors. this means they run cooler and require lower voltages which is good
just unplug everything from your current motherboard and then remove the graphics card, RAM and any other cards you may have. then you need to unscrew the motherboard and remove the spacers in the case and re-align them for the screw holes on the new motherboard then screw that in.
One thing you might need is a new power supply.
That's £30 and would be perfect if your current one struggles, but give it a go first.
Finally if you wanted to save some cash then opt for 512mb ram, but make sure its a reputable brand, same goes with the power supply if you choose to get one.