There are so many variables there, and really software environment (not dev environment, but for performance) is every bit as important.
As for stable, if you build your own PC, make sure everything matches... not just overall RAM speed and type, but timings (if you really want stable most mobos have a qualified vendor list of known RAM modules that work). Never use ram with different timings (if you add more ram, add the exact same module or replace old ram with more new ram and sell the old...). Dont skimp on power, get a trusted PSU that is 100watts over what you expect to draw. Also dont forget to run power through power conditioner as well as UPS when at a gig (laptop has internal battery so no need if your just VJing with a laptop), nothing worse than restart mid gig (has happened to me at little club party with friends years ago, lesson learned).
As for GPU, I suggest getting the modern popular performance card over getting the fastest of that version, the fastest cards are often just overclocked normal chips, and the drivers for those cards are more specific than the drivers for the standard of the card... I guess this is more for NVIDIA designed cards, cause if I was to buy a new card right now, I would invest in ATI 6990 due to its better eyefinity support over a 6950..
Now make sure your machine stays cool enough to operate properly, fans fans fans, pull cool from front, push hot out back... Get a good hardware temp sensor software, and view the temps after a few hours of prime95. <- that reminds me, I still run prime95 for at least a day as a kind of 'bake in' for the system. most parts are prebaked when shipped these days, but this will also expose any possible errors that might arise (temps too high under load, bad memory, etc).
And last, watch out for shady new SSDs, some chips are really solid and way fast, some are no good. Do lots of research on the specific drive before you buy. (I remember reading about some resolume user who a bad ssd caused his machine to reboot...). I hope some of these reminders help, also don't forget to lurk over at tomshardware.com
Asus build - intel 2.8GHz T9600, 4GB 800Mhz DDR2, ATI 4670
Rack unit - intel 2.7GHz q6600, 8GB 1066Mhz DDR2, 9800GTX+, intensity HDMI input card