I'm considering the X51 as one of my options, and from reading the OP it seems like I would indeed be able to upgrade the graphics if I needed to do so say, a year or two from now. How hard/easy is it compared to a standard desktop though? What if I had to upgrade the PSU as well? Is the X51 really inflexible compared to other systems when it comes to upgrades?
I am very much in the same situation and the more I look into this the more the X51 starts looking like a very viable option.
As many folks will confirm you can get better value for money than the X51 offers. However having said that the X51 is not badly priced at all. Currently in the UK the top option (without monitor/Blu Ray) is standing at £899 and there are vouchers available to reduce that by 10%.
If you are considering 1080p gaming then I reckon the X51 is not that badly priced at all to be honest considering the very neat form factor here.
The standard GTX 555 card is pretty easily replaced with a Radeon 7870 which is one heck of an upgrade if you consider what those are going to cost by the time you actually come to make the jump.
It is pretty clear that an SSD can be slotted on top of the existing 2.5 drive as well, so that was something else I was considering for a later date.
I am getting more and more tempted to be honest.