They will work just fine in your current motherboard. You should try to get a matched pair of X5365 but recently some folks have said that matching isn't that important. Do some searching here as some X5565 chips have different features and are identifiable by lettering on the chip like SLEG or something. Check that out before buying.
Also the 8 total cores (instead of your current 4) won't feel any faster "under your mouse". The OS and most programs will feel exactly the same except when you set them to task. For example if you're ripping a DVD or CD using
Handbrake, when you click Start it will finish in close to half the time as your current system. Same with 3D CG applications; everything will go at the same speed until you click Render and then it will finish in half the time. There's almost no video or photo editors which can take advantage of this though - in most cases the best you will get is a 10 to 20% increase for long heavy jobs. And so on.
There are very many step tutorials on-line which explain exactly how install the new CPUs once acquired. I think there's even one by me floating around somewhere. All the one's I've seen will work fine - I've never read one I didn't agree would work fine tho some are more anal than others. The most critical step is the application of thermal paste and the two important points there are 1) make the surfaces clean with acetone first, 2) don't put too much paste, and 3) keep your fingers out of the paste and don't let debris get into it.
On the video cards fouel and spaz8 are talking about, yes, they work in your MP1,1 just fine. Flash them if you want a boot screen and don't if you don't care. They're right that you won't feel much difference for just OS stuff. But you will if you're video editing or playing games. For example most games will feel 5 or 6 times faster using a GTX570 over your current X1900. Some games that were barely playable will now be very playable and games where you had to turn off certain rendering features and use low res screens can now be played at your monitor's native resolution with everything turned on. And etc. like that.
