I value and welcome your opinion though, however I'm already stretching an extra £20 for the 290 tri-x (if it fits!)
You are very wise - there should always be a cutoff point, but sometimes it's hard not to look at the next rung up the ladder. I think the best thing is that you found you can stretch for a 290 tri x over a 280x. The former is noticeably more of a beast, though the 280x is still a good card. Good luck with the build and hope the 290 fits your case.