Mining at this point is never a good idea. Especially with a sizable investment. It's risky and doesn't do anything to help the community or development of Bitcoin as a currency.
This could be easily argued by continuing on the subject of increasing network security, but I won't get into that...
"sizeable" is different for different people
I ordered a bASIC for $1069 that will give 54Ghash
I have previously spent over $1000 for 3 GFX cards that have given me 1Ghash (no I didnt buy the cheapest cards - but possibly the quietest cards - my hearing is important to me
)
I was running 14 Gfx cards here at one time then I sold some 6950s (with the coins they mined and what I sold them for they are well paid for and profitable) and bought some 7970s also coins mined + resale makes them profitable
I think I had a 'sizeable" investment in gfx cards
I think $1069 is a "reasonable" investment to continue mining
I am thinking of ordering a second and for me would still be "reasonable"
My power bill will drop an unreasonable amount when I can turn off the GFX cards - the minister for war and finance will be wrapped
( even though I have been paying my power bill through spendbitcoins with BTC)
every one should do their own due diligence and choose their way - no 2 of us are in the same situation
good luck with your decisions
Graet