Your E4500 requires an LGA775 socket and a 800mhz front side bus (FSB).
The motherboard (P5KPL-AM) has this (it even have a 1333mhz FSB). So yes, this will work.
However, when switching motherboard, you have to remove the processor from the old and insert it into the new one, along with new cool paste (or whatever it's called).
I've only used the standard paste that come with processors, already applied to the heat sink. So I don't have any suggestions on what you should buy.
But if money isn't an issue I would suggest you'd buy a new processor instead, for example the E8400 for a cure 2 duo, seeing as the current one is a bit.. dated.
And as Sprite already mentioned, you won't get full use of 6GB of memory if you don't have a 64bit OS (not limited to 64bit Vista).
But anyway, why do you need 6Gb of memory? Is this a server you're upgrading?! My computer handles Photoshop and plays HD movies, which is pretty much all I ever use, at ease with only 2Gb of memory.
Vista takes up about 1gb for me, so I still have 1gb left for other things, which is plenty for what I do.
For gaming, 4gb of memory should be more than sufficient on Vista!
But this also depends on what type of memory you buy.
Say 2 gigs of 1333mhz DDR3 memory outperforms, or equals, 4gb of 800mhz DDR2.
Also the motherboard you found (and posted in your latest post) only supports two slots, so you'd have to use two 3Gb bricks (which don't exist (I, for one, have never heard of it anyway)).
Also, the 8xxxx series from nvidia usually takes up two slots, so you'd end up with only one extra PCI slot on that mATX motherboard.
And finally, you'd probably have to buy a new power supply unit as well, considering the 8-series from nvidia requires a 6 (or
pin PCI-E power cable to even function.
