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have now gone over to dual core, so if you do see single core processors still advertised, you will probably be seeing 'outdated'/'low end' architectures or old stock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...
