I'm looking at some of the gaming PC's on NCIX.com and the ones that support DDR3 are at least about $400 more than the most expensive DDR2 PC's(when other factors are ignored). Is it worth upgrading to the next higher class PC just for the DDR3 memory or is DDR2 enough for a middle-class gaming PC?
Specific answer is NOT NOW for middle-low end systems
There are many benchmark tests that demonstrated the really small difference between DDR2 and DDR3.
This is one review made by a well-known site:
But consider that probably for $400 more there are other things to be considered? Better CPU, GPU, Motherboard, etc.
Think well to what are your today and future needs.
You aren't just paying the extra for the DDR3 memory (though it is more expensive than DDR2 by a fair chunk at the moment) you are also paying for the more expensive motherboard chipset with relevant support and a newer shinier CPU design. Also, as DDR3 is still considered high-end kit you'll probably find you are getting other higher end kit in the machine too (better GFX card, more expensive cooling components to cope with all of the above, bigger & faster drives, and so on).
You are also paying the "new tech" premium, so you will not be getting as good "value for money" by most measures - it is highly unlikely that you'll see a 30% increase in any benchmark score or subjective test in exchange for paying the 30% price hike.
For a mid-class gaming PC (ignoring that fact that "mid class" is a subjective term and assuming that it means the same to me as it does to you) I would say that a DDR2 system would be more than sufficient. For a single anecdotal data point, my 18+ month old AMD X2 with a not overclocked 3850 can play HL2Ep2 at my monitor's natural res of 1680x1050 with reasonable AA and AF settings, and you can get a noticeably faster system than mine while staying in the DDR2 arena. I would guess that a faster graphics card (I plan to get a 4870 or similar when I get a larger monitor that does 1920x1200 as I think the 3850 will show its limits more obviously in some cases at that point) will make far more difference then a jump in main-board memory tech, and now more games are taking advantage of multiple cores going quad might make more difference too.
Of course, if you want the fastest of the fast and money is no object, then a high-end DDR3 based system with all the other bells and whistles is the way to go, and you'll be able to play Crysis at full-high with AA and AF maxed at full-hd resolution at playable frame-rates. But I wouldn't call that "mid class"...
This is highly subjective. Whether DDR3 is worth it depends greatly on the situation. DDR3 has a potential data rate twice that of DDR2. It is also 30% more power efficient. But unless you couple this with a higher end processor you'd be unlikely to see the performance benefits.
High-end system design is a balancing act. CPU, GPU, RAM, system bus and hard drive all play a role in the overall performance. If any of these isn't up to the task you won't see the performance you'd expect.
If I were you I'd base my purchasing decision on my budget and real-world performance data from trade magazine reviews of similarly configured systems. The manufacturers have a vested interest in inflating their performance numbers.