I just got a lovely new Dell Latitude E4300 with Vista 64bit. I bought it with 4GB of DDR3 memory (2x2GB,1066MHZ,256X64,8K,200) and obviously, would love to bump it to 8GB. These are the specs on the memory config from Dell:
Dual Channel DD3 Memory
Two Memory slots offering up to 8GB (8GB requires 64-Bit Vista® )
Memory Bandwidth: 1066Mhz
Dell does not currently have any 4GB chips verified for this machine yet, but I found these listed:
The link you have posted is for DDR2. You system is compatible with DDR3, DDR2 memory is not compatible with your system. I would venture to guess 2 x 4GB DDR3 for your machine would cost a conssiderable amount of money.
Kingston doesn't carry 2 x 4GB kits for your system:
DDR2 and DDR3 are physically not compatible. So the answer is no, you can't use the DDR2 because it won't fit. Regarding the use of DDR3 in a Laptop, it's more expense but a good deal since DDR3 requires less voltage to run. Less voltage use means a little less heat and a little longer battery life.
Your OS will certainly use 8GB (and much more) were it available. The question is if the motherboard will drive that much. The blurb you posted indicates it will, but in a lot of cases you only find out once you try it. Regarding sourcing, what you can do is check with sites like Crucial (mass market), or Corsair & OCZ (considered higher end) to see if they have anything compatible.
One thing to consider - I've never played with the model you have - and you can easily check by flipping it over, removing the cover from the memory and looking... But many/most lappys have two slots for RAM and it appears you already have two 2GB cards... So if you wanted to go to 8GB, it wouldn't surprise me if you were forced to ditch/sell the two modules you have and buy a pair of 4GB modules. And yes, I think it's interesting that Dell say 'up to 8GB', yet don't offer 4GB modules...