In my opinion this is a little too hard to predict at the moment ...
This is not meant to be an overly accurate chart, but if you use the XBox 360/PS3's processing power as being 1X then this is what I expect you'd be able to sell a system for with relative processing power in a given year:
|-||4X to 8X||8X to 16X||16X to 32X|
|2010||$300 to $500||$500 to $800||$800 to $1,200|
|2011||$200 to $400||$400 to $600||$600 to $1,000|
|2012||$150 to $300||$300 to $400||$400 to $600|
|2013||$100 to $200||$200 to $300||$300 to $500|
Depending on the strategy the companies choose to go forward with will largely determine what year they release in; if Sony and Microsoft really want to push the boundries on graphical improvements they will need to get into the 16X to 32X range, which will require either a later release date or a higher price tag.
In my opinion Nintendo has a massive advantage over Sony and Microsoft. In 2011 Nintendo can release a system which is dramatically more impressive than the Wii (noticeably better than the XBox 360 and PS3) at a price which is similar to what the PS3 and XBox 360 will be charging at the time. To a certain extent this puts Microsoft and Sony in a tough position because they can either stick to a modest upgrade, they wait and give Nintendo an opportunity to build a solid userbase, or they produce an expensive system; the modest upgrade can work if you supply an alternative reason to upgrade, waiting can work if Nintendo fails to use the first mover advantage, and having an expensive system can work if you provide adequate reason to pay extra money.
At the moment I would expect the XBox 360 to release a moderately powerful (8x to 16x) system in 2011 for $400 to $500, Nintendo to produce a less powerful system (4x to 8x) system in 2011 for $200 to $300 and Sony to hold out until 2012 or 2013 at sell a high powered (16X to 32X) system for $400 to $600. I expect the XBox 360 will die soon after the release of Microsoft's follow-up system, the Wii will sell well for a couple of years after Nintendo releases a follow up system due to its low price ($75) and the PS3 will be on life support after Nintendo and Microsoft release their systems.