One large ram module or two smaller ones?

your concern for full ram usage may not be a problem since Modern operating systems use RAM as a file cache to speed things up. Assuming your computer is performing well, there’s nothing to worry about.The article here explains why It’s Good That Your Computer’s RAM Is Full.You may want to upgrade your RAM depending upon the processor you use.if you are using a 32 bit processor that don't support PAE, don't worry , 4gb of RAM might be enough for you.you may refer RAM limit.Physical Address Extension(PAE) allows a 32-bit processor to access up to 64GB of RAM given that os supports it. otherwise , if you use x64 processors ,"more the merrier".i.e,theoretically a 64-bit architecture can address 16.8 million terabytes of memory, or 2^64 bytes.

SUMMARY:To get most out of your RAM (> 4gb) you must be using 64-bit computer and operating system.

IMHO 8gb of memory is more than enough to run many programs unless you are a hard core gamer / video editor.

For dual channel configuration and best performance DIMMs (dual inline memory module) need to be installed in pairs.and hence it's best to have 2x 4GB RAM modules instaed of 1 8GB RAM module.

i.e,one can place a matched pair of memory modules in a bank, but a different-capacity pair of modules in the other bank, as long as they are of the same speed. Modules rated at different speeds can be run in dual-channel mode, although the motherboard will then run all memory modules at the speed of the slowest module. It is advised not to put different size and speed of ram into a bank.

Adressing your concern, you can put two 4gb RAM modules of same speed in one bank and leave the other bank empty. or you can have a 4gb RAM module in one bank and 2 gb RAM module in other bank.But you sholdn't use a 4gb RAM module and a 2 gb RAM module in one bank.

as you say , it's not a good idea to have unequal amount of ram in a bank.

If you have a dual-channel-capable motherboard, then the best option is always having two equal modules so your hardware could take advantage of this feature which allows simultaneous access to memory, reducing latency.

The first question you need to consider before the query you have put above is "what is the maximum amount of memory your laptop supports?" If your laptop supports 16GB or more, then single 8GB module will work and that's great news else it may not work unless your laptop has a single slot.

Yes, it is sometime not good to have unequal amount of ram in a bank and you may struggle to get the memory to work together although they may be of the same speed as laptops can be very picky sometimes. But in most cases, I've seen the unequal amount of ram work just fine like the other combos.

Well IMO, the 8GB-empty is the better choice than 4GB-4GBas you will run the same capacity with less slots (which gives you the advantage of not having to throw the memory away in case you wanna upgrade in the future) and it runs with less stress to the memory controller. In the hindsight, you will run 8GB-emptyin single channel mode rather than dual channel mode. Dual channel has the advantage over single channel as your memory's performance will improve and performance difference can be almost 10%-15%. So in the end it comes to your preference I believe.

So if you are not sure how much memory your laptop supports then you can try the memory scanner software from the one of the sites like crucial, Kingston etc or this one http://www.memorystock.com/ (click on scan your computer button for all the info about your laptop and the compatible memory upgrade for the same).

P.S - Make sure you have a 64Bit OS if you are trying to install above 4GB of memory.