I´m using VST link. I´m using a MAC (Cubase 5.5) for master and PC (Cubase 4) for slave. (Also works with two PC´s). The set up is pretty clearly explained in the manual. I use a Hammerfall audio interface in the MAC and a Creamware in the PC. I use the ADAT connection with fiber optical cables to connect the two audio interfaces. But this is just one of several ways to connect.
Now, another and perhaps a better way you should consider, is to buy VEPro and connect the two computers using LAN. It is like a mixer or plug within the mixer/plug. (Then you do not need the second audio interface).
You install VEPro on both PC´s and you can use the second PC through VEPro on your master PC. VEPro will behave like a plug in for both instruments and FX. VEPro does not support all software out there, but most of the modern plugs. (two dongles, one on each machine needed. You do however not need to load Cubase on the slave, only VEPro).
For VST link I need to route the audio signal (bus out) from the slave to a mixer, and record to master from there. VEPro does not need this set up. Signals is within your DAW.
I would NOT go for the 64 bit DAW upgrade. It will give you more memory access yes, but might cause compatibility problems on many of your plugs. (Waves for one). I have all the facilities I need to run both Cubase and Logic in 64 bit, but choose to run both in 32. VEPro is great here as well. You can use it in a 32 bit DAW, but that plug will give you 64 bit access to whatever plug you put into it (as long as it supports 64 bit), thus you can add memory without the 64 upgrade on your DAW. So I use VEPro in my 32 bit projects, but I can access Omnisphere, Trilian, PLAY, Kontakt in 64 bit through VEPro. I use VST link to run older software that VEPro does not support (like Creamware synths and Steinbergs prequel to Halion; USM that will run under Cubase 4 but not 5), but i mostly use VEPro for “system expansion”.