Hi Mamatilly - bulbs in the green are usually delivered after they have flowered, so they will have leaves but no flowers until spring 2012! Snowdrops you do really need to get in the green otherwise they dry out really quickly once they are lifted and won't grow well. So I would avoid the catalogue which doesn't have them as in the green.
Bulbs like snowdrops, which you want to naturalise, will produce small offset bulbs when they have settled in. The offset bulbs are small "baby" bulbs which form off the parent bulb. These will then take a year or two (depending on variety) to get to flowering size and this is how bulbs naturalise and increase.
I think you will be alright with bluebells as bulbs rather than in the green - I've had success this way previously.
It is really worth your while shopping around (search on Google) as prices can vary enormously. Also, you want to go for the ordinary single snowdrop Galanthus nivalis. You should be able to buy these in 50s and 100s. There are some very rare and expensive snowdrop varieties which can be quite a collector's item, and sell for ridiculous sums of money.
When you do get your snowdrops, you will need to lift the clumps every couple of years and divide them up and plant them into more clumps, otherwise the clumps get very congested and flowering suffers.
You could put crocus in your lawn - some varieties (tommasianus etc) will naturalise well and you can plant these as dry bulbs. Just make some holes in teh lawn with a fork and drop them in!
Hope this helps!