What does installing new X2 plug-ins do to my current plug-ins??

1) Well, they are updated versions and 64 bit variants are now available if you have a 64-bit OS but 32 bit versions of the plugins.  If you wish to keep your old versions, I'd not install Rapture as part of the main install (i.e. de-select it) and install that separately to chose a different location; there is a stand-alone installer available from Cakewalk's website for the updated Rapture.  Dim Pro has its own installer so you can easily select a new location for that (or not install it).
 
However, personally I've had issues with Rapture in particular where after installing the updated version, the older version re-asked for its registration key and would not accept it.  I would guess this is due to registry entries issues.  So, in my personal experience, I've not been able to have both the older and later versions of Rapture installed together.  Note also that if you update it, all your existing projects using Rapture will most likely report than they are unable to find Rapture.
 
I'd say if you're happy with the current versions, keep them.  If you decide to update to later/64-bit versions you always have that option but you might first want to take note of any patch names in existing projects so you can transition them to the new versions of the plugins.
 
2) I believe that it just doesn't install the new item (i.e. nothing is removed).
 
HTH

Saw your post and wanted to try and help. But I wasn't quite sure what the facts were, so instead of giving wrong or not clear info, I didn't respond. However I kept thinking about it and started to remember what happened to me when I upgraded to X2 with Dim Pro 1.5 and Rapture already installed.
 
1) If you already have Dim Pro and Rapture installed, no need to install the versions with X2. They are the same.
[edit] I forgot about 32/64bit versions. And ston makes a good point. [/edit]
Now there is some new content for Dim Pro that comes with X2 IIRC. You can install just that.
 
Might want to look at these two KB articles:
http://www.cakewalk.com/Support/kb/reader.aspx/2007013203
http://www.cakewalk.com/Support/kb/reader.aspx/2007013239
 
In my case I had to re-enter my Reg code when I tried to use Dim Pro after the X2 upgrade. And I couldn't remember which version I installed. The one that I originally bought, the one from X1 or X2. Well as it would be, I tried them all! I ended up using the one from my original purchase. I got that code from the store My Account. That was the only one that would work.
 
2) From any previous install of Cakewalk products that I have done, the installation routine has never "removed" something I already had installed when I didn't check it. So your ok there. CW is excellent about having other versions of Sonar installed all at the same time. Many of us do that.

Grem
I have X1 and X2 installed on the same machine. X2 install did not do anything to the X1 install.

Ah, I stand corrected!  I'll remove that piece of nonsense from my post.

As you already had 64-bit X1, I'd guess that would have installed 64 bit versions of the other plugins and synths (I went from 8.5 -> X2 so don't know a lot about X1). 
 
Last time I installed X2, I selected the option which let me choose which components to install.  I noticed a bunch of what looked like 'legacy' stuff that isn't selected for install by default, so in the end I just said 'sod it' and installed everything including the kitchen sink.
 
The install instructions say: "please run Part1.exe of each installer".  So I would say that in addition to 02_SONAR_X2_Producer.part1.exe, you will also need to run 03_Loops_And_One_Shots.part1.exe, 05_SONAR_X2_Producer_Additional_Content.exe and then the Dim Pro and its additional content files if desired.
 
The worst thing that'll happen is you'll spend a few minutes overwriting existing files.