You can comfort yourself with the fact that Macs seem to hold value well, and that if you buy now only to see a major upgrade in the next few months, you can recover a good portion of your investment towards purchasing the next one. I own a 20" C2D imac and love it.
That's not entirely true.
When you go to sell your iMac later, it's configuration will be compared against the configuration currently available and the price of the new model with that configuration. Additionally, many will also compare your asking price against the same model being sold as a refurb from Apple (the refurb would have a full warranty, and may even be a model newer than yours for likely less money).
So, the newer model will automatically drop the perceived value of your machine. And, then you'll have the additional value loss due to it being used, having less warranty remaining, and the possibility of other damage.
For example, I purchased my first iMac G5 1.8 GHz 17-inch shortly before they introduced the revision B systems.
I had invested about $2200 into my machine to get it equipped as delivered. $400 of that was memory.
But, when Apple introduced the second revision a few months later, they made a lot of the build-to-order options that I chose standard equipment on the new model. Additionally, they also increased the standard hard drive size of the standard model.
Additionally, the price of memory had dropped by $200 in that couple of months. So, my machine suffered a pretty substantial and pretty fast depreciation.
For various reasons, it became necessary to sell the machine at that time. And, because the new model had included all the build-to-order options I chose, and because the new model actually exceeded the specs of my customized model to some extent, and because the price of memory had dropped so significantly, and because the new model with all that stuff could be had for $1200 ($1400 if you included the memory), my machine was worth very little.
Since it was in the classification of being used, that further decreased it's value.
The absolute best offer I could get on the machine was $1200 for everything. The average offer was around $800 to $900.
So, in roughly 3 months, the machines value dropped from $2200 to $1200. That's not exactly what I would call holding it's value.
By the time the revision C iMac G5 came out, people could hardly give away the first iMac G5 models.
Compare that to the old days before Apple's rapid product replacement schedules:
I purchased my PowerMac G3-266 DT with DVD and Video Capture for about $1500 brand-new the week they were released. I sold it about 5 years later for around $450. So, it depreciated $1000 in 5 years instead of 3 months.
And, with that machine, I hadn't even bumped the specs. It was sold exactly as it arrived when I purchased it.
I purchased an iBook G3 Clamshell 333 MHz right after it had been discontinued (purchased brand-new). Got it for $900 since it was discontinued. It came with two extra batteries. I sold it around 4 years later. I got $475 for it. So, it had only depreciated around $400 in 4 years time.
Many people are still stuck in the past thinking that the Macs hold their value for years and years. But, that's just not true anymore.
With the increased rate at which Apple decreases the price for a particular configuration during the last couple of years, the retained value of the previous models drop faster. That's because the new model usually sells for the same price, but includes more items that were build-to-order options on the immediately previous model.
So, while the price of the machine remains about the same, the price of the package is generally cheaper. Combine that with your machine being used. And, the fact that Apple will offer a refurb for less than what you'd like to get, and offer a full warranty on it, and selling second-hand is a lot harder than it used to be.
If I can buy the same model iMac as a refurb, and it comes with a full warranty, and it's already $600 off the previous price. Then, I'm very likely to go that route as opposed to buying the same system from a private party with a lesser warranty remaining and the additional risk that the past owner didn't take care of it.
With the market as it is today, both current PC's and Macs will retain their value to the same approximate level. The only difference is that the PC generally has a cheaper starting price. So, it's depreciation will complete sooner. But, if my $500 machine depreciates to $200, that doesn't really bother me as much as if my $2000 machine depreciates to $1000 in the same time period.
