If I get the urge to use W7 or get bored, do I cannabilise my present rig or build a new one with 64 bit?
It would depend on how old your present machine is, as to whether or not it is worth upgrading. If older than 4 -5 years, I would go with building a new system. Also, some motherboards can not be upgraded on. For example, you can no longer buy a new CPU for a 939 socket mobo, so it would be wise to check on availability and cost for all parts (CPU/RAM/GPU, etc) for your current mobo... as in older SD and DDR1 RAM is more expensive than the much newer DDR2 & DDR3 RAM.
The other consideration when upgrading is the power supply. Often the old PSU is not adequate and needs to be replaced, so at the end of the day it may be more economical to just build a new machine.
Having said that, Windows 7 is nowhere near as resource hungry as Vista and it will run well on lower end PC's, whereas Vista caused even some mid to upper range PC's to struggle. I ran Win7 Ultimate x64 RC on a Pentium 4 @ 2.8ghz with 2gb DDR 400 for a while and it performed quite well... not superbly, perhaps, but certainly well enough for average home use.
In conclusion, I would recommend going with Win 7 x64. The driver support for Win7 is excellent and it will run just about any program you can throw at it, not to mention that it utilises hardware better and can access more than 4gb RAM, which is handy when multi-tasking.
