I would suggest mining at cex.io its pretty cheap and you start mining within minutes and considering how fast the price is increasing right now its well worth it
Do not do Cex.io. Any "share" is basically guaranteed to depreciate, so you'll likely never be able to buy a GH and sell it back for any higher than you paid. Plus right now individual GH's are worth a pittance for actual mining, ~36GH will generate around .03 BTC/day (if you are lucky), right now that'd cost around 2.88
BTC. With depreciation, again, you'll be lucky to get back what you pay in, even with mining dividends.
Mining is very tricky right now. Definitely look to purchase in fiat (USD, Euro) and measure 'profit' in the BTC generated. More or less anyone charging a set BTC price will benefit a lot more than you when purchasing hardware from them, so look for something that is fixed price. For example, folks are selling the AM cubes for 1.75
BTC. In early October that was ~$250, as of right now on Gox prices it is $1155. If possible, also, when purchasing (in fiat!) try to make sure you are purchasing via Paypal or a credit card so you have a company to fall back on if your vendor/reseller doesn't deliver or refuses a refund when they aren't providing product.
The advice about Scrypt is solid too, if you already have hardware and can add an AMD 7970/7950 to it you'll do much better than purchasing any of the "shipping" asics for BTC right now.