How many keywords is enough

Hi Guys,

I have started PPC campaigns for laptops, digital cameras, lcd's and external hard drives, etc. For each of these, i broke them down in to further ad groups. Example, i have iomega external hard drives, seagate and western digital as a separate campaigns. I then further broke it down into separate adgroups per campaign - 100-200GB, 250-500Gb, 750GB-1TB. Each of these adgroups has keywords specific to that adgroup from your two word keywords to your longtail 3/4 word keyphrases.

My question is this, i have tons of keywords for each adgroup almost 70. is that enough or should i get more or should i cut them down? Should i set up separate ad grops with the same keywords but different matches so i have all the same keywords but different matches (exact, phrase, broad). At present, my long tail vague keywords like "160GB iomega external hdd price" as a keyphrase is set on broad or should i set those types to phrase.

Also, is there any rule of thumb or guideline as to which match type i should associate keyphrases to. 2 word phrases should be match, 3/4 should be broad or exact, so on?

what is the best tactic in getting negative keywords? Also, is there any ppc software that is free yet powerful.

I'd really appreciate all the help guys!