You should try to maximize the pc you have now. There are a few things you can do to improve your performance.
Make sure you are not running any non-audio and non-essential applications as they require memory that won't be available to Cubase. There are backround applications(like find fast) that could start doing disk scans in the middle of a session.
Check out your Close program dialog box(Ctl-Alt-Del) to find out what's running. End task on what you don't need, there's nothing there that you can hurt and they'll all return when you reboot.
Make sure you have a multi-client driver that allows different applications shared access to your soundcard.
Use a seperate hard drive ONLY for the audio preferably on a different E-ide or SCSI port from your system disk. You should defrag after every session too.
Hard drives spin at speeds between 5400 rpm - 15000 rpm. The faster the better. Check out the minimal access time or track to track seek time which is how much time it takes the head to move from one cylinder to another. A small seek time is better (8-9 millisecs is good, a smaller number is better).
Also check out the sustained data transfer rate. The peak data transfer rate don't mean much.
If your E-IDE hard drive uses DMA or Ultra DMA technology make sure to enable this in your system configuration to improve it's performance.
You can change some settings on your video diplay to improve your performance. If your using a fast gaming card or a video adapter with TV inputs trash them. There's less chance of a problem with an AGP card.
Remove the screen saver option and just turn your screen off when you don't need it. Reduce your graphic acceleration gradually to see if things improve, if not fully reenable it. Reduce the color display to 16 or even 8 bit and reduce the colors to 256(big drag if you play video games, paint or view photos.)
Remove all the wallpaper images from your desktop.
If all that don't work then get a freakin mac
What I've learned to do is to some sub mixes on large projects and that seems to work real well. G/L