DIMENSION - MULTIPLE INSTRUMENTS WITHOUT MULTIPLE INSERTIONS ?

DimPro/LE is pretty much one instrument-one track. The vast majority of soft synths will behave this way, however there are exceptions, such as TTS-1, SampleTank, and Proteus.
The synth GUI will generally make it obvious whether or not a synth supports multiple tracks within one instance.
With contemporary computers, multiple instances should not be a problem.
Even my aging P4 system does ok up to a point.

You can do multiple instances with DimPro/LE. You just can't get several instruments form one instance like you can with TTS-1.
I am also using an XP system that is a bit long in the tooth, but it still does ok. See my specs below.
RAM is very important when you have several synths loaded. If you don't have enough, you can run into problems.
I have 1 Gb, which can get a little tight on a big project. 2 Gb would be significantly better. XP (32 bit) supports  up to 3 Gb RAM.
What are your system specs? If you can include them in you signature, that helps us help you.
I prefer DimPro to Dim LE, because it offers much more depth of control, as well as a much larger sample library.
In my experience, most sample libraries have a few dogs, but for the most part, I am quite pleased with the sound of DimPro.
This particular tune includes sounds from DimPro, Sampletank, and Studio Instruments.
http://soundclick.com/share.cfm?id=10285834

If you freeze synth tracks that you are not actively working with, you can minimize the stress on your system.
Also, when you are mixing, you can back your latency way out, to say, 100 ms. This gives the CPU a little breathing room.

TTS-1 has a tambourine, and is very memory/cpu efficient, and is 16 part multi-timbral, outputting to 4 separate output channels

The sounds aren't as polished as Dimension Pro, but quite useable