The best answer is buy a Bluray player, and buy or borrow a couple of the double/triple play BD/DVD packages that bendrummond mentions, or a couple of BDs you already have the DVD of.
Then try a few A/B comparisons at your standard viewing distance. It's not a question of whether the BD image will be better than the DVD - it will - but whether the premium you pay makes the difference worth it to you. Remember you may get a bigger screen in the lifetime of the disc, so give BD the benefit of any doubt.
FWIW, on my setup, 42" 1080p plasma at 2.5-3m, I'd always buy BD if:
(a) 3D movie on 3D TV - obvious
(b) high quality digital source - Pixar, Avatar 2D
(c) loving restoration - Disney, Warner classics, Sony James Bond 4K2K masters
(b) better version - CE3K includes three versions, Blade Runner TFC, Dark City DC...
and always buy DVD unless the BD was as cheap or cheaper if:
(a) one-watch stuff - e.g., Nic Cage in a bad wig...
(b) TV series boxsets, especially if 4:3
Remember also that a DVD will play on portables, and can be easily converted for media players, the BD won't and can't.