i) GHz isn't a useful measurement of the application speed of processors.

ii) The most popular series of processors have now gone over to dual core, so if you do see single core processors still advertised, you will probably be seeing 'outdated'/'low end' architectures or old stock

iii) Dual core processors can be 'as fast' as single core processors at lower clock speeds. Whether this scaling is anywhere like linear depends very heavily on whether your application was written to take advantage of the multiple cores (multi-threaded). The vendor of your target software will have a better idea of this than I do, since I don't know what software it is that you are proposing. It would be odd if the vendor will only tell you whether obsolescent hardware is adequate for their application.

More specifically, if you are seeing 'Pentium Dxxx' processors (which are already dual core), be aware that Core2Duo processors which have equivalent performance are those with roughly half the clock speed. (Which should give you a clue about how seriously you should take GHz as a direct indicator of performance).

Also, be aware that if you are looking at AMD Sempron processors, that these are intended for the low end/economy market, have smaller caches, and while they may have been competitive once, these days the comparison with even the lowest of the core2duos is, errr, not in favour of the Semprons (nor even close). Also note that if you see a Sempron 3400, it does NOT mean that the Sempron has a clock of 3.4 GHz. It means that it has the equivalent performance of a non-existant Celeron if it could have scaled to 3.4 GHz (as guesstimated by AMD). This is probably perfectly adequate for many office tasks, but you are asking about performance...

I'd like to add to Virtual's advice and differ in one respect.

GHz matters. If you run a single threaded program in a 2.4GHz core2 duo PC, it'll take the same time to run as a single core 2.4GHz CPU with the same cache size nd same XP.

The core2 Duos are 64 bit and best rsults for you will be with Windows XP 64 and rendering software that has a 64 bit version and is also multi-processor smart. Also, XP 64 gives you more RAM (e.g. 8MB and beyond) which significantly assists rndering software; XP (32) only allows up to 4GB RAM.

Also, for video rendering, Quad CPUs make a difference except at the highest end Intel core 2 duo (E6850).

I made the choice of an 8GB XP 64 system with Q6600 (2.4 GHz Quad); I need it for other software reasons but do plan to get round to some video rendering soon.

I wouldn't touch AMD for the reasons offered by Virtual.

Let us know what you decide.