Hi Guys... I seem to getting more and more into VST instruments (eek!) and wondering about the RAM constraints. My OS is XP Pro which limits me to 4 Gb RAM. Does anyone know roughly how many VSTs HS6 XL can handle comfortably with 4 Gb?? I hesitate to go to a faster PC and Windows 7 - with loads of RAM - 'cos of some nasty issues I've read about concerning updating drivers for my Firewire 410, Midiman 8 x 8 etc. Anyone got experience or advice to share??

It totally depends on the VSTis. Some are hungrier than others,  but as there is the freeze/bounce function, you'll always be able to add another one. And it's not only depending on the amount of RAM, your CPU power plays an equally important role (again, depending on the VSTi).

I'd say that in your present setup the CPU is an equally limiting factor as the RAM.

Try launching Task Manager and monitor the RAM usage while add more and more softsynths to a Sonar project.  You will find that loading a synth hardly has any effect, whereas loading an instrument or patch that relies on audio clip samples certainly does increase the amount of RAM used.  Try going over 2GB and see what happens.

...and actually playing the loaded  synth can hit the CPU heavily.

Kev and Kalle are on the money.
Your CPU is a bigger factor than RAM, especially with 3 or 4 Gb available.
Launch Task Manager. It gives you a more accurate view of CPU activity than the CPU meter in SHS6.
Even on my modest system, I am able to load a surprising number of VSTs.
As noted, this will vary depending on which VSTis I am using.
I like to track softsynths live, so low latency is important at the recording stage. I'll typically have two or three synths active.
As you add more synths, the CPU will have a harder time keeping up with the load, and you may start to hear some pops and crackles. Temporarily freeze the synths you are not working with.
Once you have your tracks recorded, you can increase the latency, and unfreeze all of your synths.
As an example, Khyber Pass9 has 11 synths, plus several EQs, reverbs, and Boost 11.
At 9ms, this would have guaranteed a dropout, but at 200ms, it was smooth sailing.
Just keep an eye on those meters, and use good management techniques.
SHS6 is a strong performer under XP, and the program itself should not be a limiting factor.

Well guys... What a wealth of information you have provided me - and an education to go with it. I have access to a CPU upgrade and I might as well go to 4 GB at the same time. And stick to XP for the moment!! Thank you all for your inputs, your time and help Mike

Your 32 bit XP can use about 3,3 Gb of RAM, so you won't get much improvement by adding 1 Gb, but of course, more is more, even if it is only a little bit more.