I also use my computer for running cubse sx3 and i can tell you now that the upgrading of your ram will not completly solve the cpu spiking.
You have way too many background services running at the same time.
When you are using your comp for recording,, you have email, explorer, antivirus etc,,etc,
These services will all cause conflict with the cpu as they will all be trying to grab as much % of your cpu as they can,, along with your recording software and all your vst pluggins.
The only way to overcome this permenantly is to tweak your xp so it acts more like a DAW, giving preference to you music making above all else. But unfortunatly,,,, this will be at the expence of all your other services.
There are a few options open to you ,,such as dual booting your comp or partitioning the drive so you use 1 boot for all your recording and vst storage,, and the other for your domestic use ie,,internet etc,,,
If you have reached a point in your recording career where it has becoming more than a passing hobby, then you really must consider getting another computer and using that for your recording.
I have been thru all of this before,,,, beleive me,,,, its time to get another comp, use one for your internet and everthing else you do domestically,, and the other, solesly for your recording.
There really isnt any other way to obtain near proffessional results. you will continually overload your cpu,, your recordings will start to suffer quality issues,, clicks, drop outs and glitches.
I am a member of CUBASE.NET forum, soley for cubase users, but there are litterally hundreds of threads all about tweaking xp,, partitioning a drive, setting up a dual boot,, building or buying a DAW,,,,,
Its the place to go,,,,,,,, browse the forum,, read the posts,,, you will soon get a better idea of what you need to start doing to contiue recording glitch free.
I do hope this has been of some help to you,,, I have been there,,, i now have 2 computers,, end of story.
Good luck,,,, kev.