So... Last night, right after submitting my response, I fired up Cubase and made a little electro-ish track using nothing but synth-type VSTi's (no samplers at all).
AMD Athlon XP2000+
MSI K7 Master-S motherboard (Onboard U160 SCSI)
1GB Kingston RAM
M-Audio Audiophile 192
I ran Cubase SX3 with 1 instance of Waldorf Attack (running 6 independent channels), 2 NI FM8 instances, and 3 Novation V-Station instances. I was barely over 50% CPU and I was still running smoothly at 6ms latency with no pops or clicks. Then I added a couple of fx, Waves True Verb, Waldorf D-Pole, and a couple of delays. Still no issues.
I did, however, manage to bring it all crashing down rather harshly.
The one VST that I have that's been a sore spot for me is NI's Massive. My laptop can handle it without a problem (Dell Inspiron 630m w/2GHz Pentium D). I even have a number of drum and bass tracks that use it without a hiccup (
http://www.soundclick.com/projectganymede - track called "Horizon" uses it). However, I can't mix any of those tracks on my desktop because I need a faster processor to do it.
So... right after I created something that I had really considered keeping and expanding on (and hopefully have released), I opened up Massive to add one sound from it to the composition.
It locked my machine up HARD after I started to record. I lost the whole track.
Moral of the Story: For sample based music, even if you use a couple of virtual synths in the form of Lounge Lizard, B3, or Korg M1/Wavestation, what you have is more than adequate. You just need more RAM to handle the sample loads. If, however, you're more into electronic music or want to use VST's that can tax the hell out of your processor, you need to upgrade the CPU (not necessarily the whole machine). These days, a fast P4 processor that will work on your PC shouldn't be more than $50. What you will need tho do, though, is take a look at Dell's website to find out the fastest processor your PC will handle and buy it. If you're lucky, you can drop in a 3GHz P4 and that should be fine for a good while.
now... after writing the above paragraph, I rolled over to Dell's site.
http://support.dell.com/support/edocs/systems/dim2400/en/sm_en/specs.htm
Your PC is capable of running a 3.06GHz P4. Find one on ebay or locally and buy it along with more RAM.