Given that the total amount of RAM on your laptop is 4 Gb, I guess the maximum RAM you should use for Rybka is 1 Gb (more hash = better performance, but doubling RAM in this range of sizes achieves only a few percent of speedup). All of the above engines are the same, and for normal chess you should use either DR4 or DR4 SSE42 (this one is for new processors which use SSE42 set of instructions, and I guess yours qualifies, so use SSE42 verson, which will make better use of processors and be faster than ordinary DR4 by say 5% or so). DR4 960 is for special chess where initial position is decided randomly (except for pawns), but as for the rest it is the same as DR4. I'm not familiar with Triple Brain function in Shredder GUI but I guess it analyses the same position with 3 different engines (provided you have 3). This can give you an interesting extra view on the position, but will reduce the total time used by DR4 (believed to be the best) to one third of the total, so it is a bit of a tradeoff.

I have a I7 quad core machine with 6gb RAM, and uses SSE42 on default settings. It operates between 200 to 300 kn/s, and has never lost a ICCF game. So . . . there you have it

i'd recommend simply using the cpu/ram usage gadget (right click on desktop & click gadgets) & see what % of RAM your OS is using. close all obvious unneccsary processes and/or download gamebooster (do a quick google search) which will do this for you & free up some RAM. unfortunately sidebar (which the RAM meter gadget runs on) is one of these processes, but after you've checked to see what the RAM situation is you can close it with either task manager (ctrl+shift+esc) or gamebooster.  the reason maximizing RAM is important isn't so much do to with hash size as with large pages, which is a new feature in rybka 4.  it'll speed up the engine by a noticeable degree.  i don't use it anymore myself, because -- and this important -- if you should switch engines, or close rybka, the large pages will not function without a reboot.  to enable large pages just go the UCI settings & tick the box next to it.  the reason RAM matters here is that there isn't much difference b/w 1 & 2GB in terms of hash size (afaik) but the larger the hash, the more benefit you will get from the large page function.  i am not sure if a 4GB machine running win 7 is capable of using 2GB hash without slowdown...if not, it's no biggie, but if you can, why not?

i would also not recommend using triple brain or other engines if you are interested only in pure performance.  however shredder is a very different engine from rybka (much slower & more knowledge based) & can at times provide some interesting alternatives.  but it is at least 100 elo weaker.

There are optimums.  The relevant factors are how much time is spent in a position on average and the kn/s rate of your machine.  Usually it is between 16MB and 256MB.  If it sits all day on one position then 512MB.  A weekend or more: 1GB.

The only way to find your optimums is to do a lot of tests.  The differences are not huge.  64 MB or 128MB are fine likely.