AFAIK there is no proof whatsoever that the Cylons were planning any kind of attack at all until they discovered that the Colonials had broken the armistice by sending a ship across the line on a spying mission. The Cylon fleet does appear to be much smaller than the Colonial one and we've got no idea whether its only purpose originally was defensive.
In fact there are indications that the Colonials actually wanted to restart the war and even that the mission might have been meant as a deliberate provocation designed to lead to a Cylon attack that could be used as the reasoning for a second Cylon war.
Actually, I kind of disagree on this.
First of all, remember what John/Cavil did with the final 5 in the show.. that was in preparation for the attack as well. The guy was never really peace-loving, he basically needed to get rid of the 5 who stopped the first war to get his way. The Cylons as a whole may not have struck, sure, but with Cavil at the helm, it was just a matter of time.
Also, the attack they pulled on the colonies had to have taken years of meticulous planning. Plant humanoid agents in key positions, gather intelligence, get a way to shut down their fleet..
Also, take a look at the design changes in their ships between the war, particularly on the basestar. The 1st war era baseship is a massive armored floating fortress that can more then challenge a battlestar in a gun battle. The new basestar introduced in the mini is an entirely different design - sure, it's big, but much more slender-built, this ship was never built to dish it out with the battlestars at close range. This ship's weapons are completely streamlined to function at long range, and coupled with 864 raider racks per ship and much weaker armor then in the previous baseship iteration, you got a vessel that was designed to surprise and overwhelm the enemy from a distance, using it's long range weapons and superior numbers of raiders. I'd characterize that as a first strike weapon.
In a long, drawn-out war you really want ships that can receive a lot of punishment and engage the enemy from every range, from short to long - such as the 1st war era baseship we saw in Razor. If you're expecting to surprise and annihilate your enemy quickly, then the modern basestar is your ship of choice. Pretty much everything we saw of the new cylon tech suggests it was designed as colonial - killers. I don't know exactly how many years before the attack that the ill-fated Valkyrie mission happened, but I'd imagine Cavil's plans were in motion long before then.
Besides, you don't go and annihilate a whole civilization just because they sent a stealth craft over your border to take a few pictures. Yes, it's a provocation, but it hardly warrants genocide. If anything, this event was probably used as an excuse by Cavil to justify his plans to his fellow Cylons.