well sx3 should support multiprocessor and upping the processor a few mhz wont make that much difference tbh, i'd go for a dualcore or higher cos even dualcore is now getting old so gives good bang for buck now that quadcore is getting mainstream.
the bang for buck core2duo used to be the Intel Core 2 Duo E6600 - with 4mb cache, but the Intel Quad Core Q6600 2.4GHz S775 is now 163 quid in UK and would seem to be a good choice for a little future-proofing.
however if you are on a real poor-ass budget I dunno, see, it depends what you want... We beleive it or not still use an old AMD XP machine for music score jobs, it's old, nowhere near as fast as the most basic dualcore, but it does the work and gets paid.... so a quadcore like that above should see you right for most typical music work, even if you get more juicey plugins in future or want to run more plugs or more hi-end CPU-hungry plugs.
so for super cheap-ass the old AMD X2 cpu's are super-value dual cores and you can build something like an AMD4000X2 for a couple of hundred quid inc a new case, or you go for a slightly more pricey Intel Core2Duo but that can be upgraded further in future than the AMD, or you go straight for a quadcore, it's all just about budget
I've only built one intel dual core tbh, but the stock fan which comes with them is:
1. Very easy to fit, childs-play in fact - and 2. Is reasonably quiet, certainly it is quieter than a stock AMD dualcore cooler
but you still might decide to add a super-quiet cooler from zalman or whoever, thats your choice and can be done at the initial build or afterwards at a future date; the point is, it's not a nightmare to live with the stock intel fan/cooler
i'd put that with an efficient pci card solution for audio i/o, and while i cant pretend to have tried every card out there, i've tried a few and the esi esp1010 (8 x in/out) or Juli@ (stereo in/out) are VERY efficient and offer very low latency drivers.
one thing tho you could try with your current machine, i'm not a fulltime Cubase user so this may not work as well as it does for Logic, but did you try this little fix:?
Open control-panel then choose:
system / advanced / performance / settings button
Now choose the ADVANCED tab / processor scheduling - set it to 'background services'
Apply and Ok it all and reboot..
see if that makes any difference, I dunno the cpu-drain those plugins you use are adding to the equation, but it's worth a try first to do that little fix, there are other little fixes and streamlining things you possibly can do with your current machine which might stop the popping, but that current machine you have is not so powerful if you want to use hungry plugins & perhaps it's time to move on up anyways.
the only downside of all this is that if you must stay with a laptop solution then it'll cost way more than building desktop machine, but of course you get the portability. If it has to be a laptop, go for a dualcore anyways.
one other thing, perhaps it's possible to borrow a different audio interface? mebbe that echo one is letting the show down
___________________________________
I had an idea for a script once. It's basically Jaws except when the guys in the boat are going after Jaws, they look around and there's an even bigger Jaws. The guys have to team up with Jaws to get Bigger Jaws.... I call it... Big Jaws!!!