If you fit 235/35/19 the gap between the tyre and suspension mount is tiny on front, so fitting 15mil spacers brings them away and also fills the arch, so the rears need fitting too or the car looks silly.
DO NOT buy cheap spacer, or you will get wheel wobble all the time, you MUST only buy Eibach hub centric spacers and get them imported (takes 2weeks).
This is were I got mine from, and Nathan (Albatross) whos car can be seen in the calendar pics, I got all my advice from him and I'm sure if you PM him he will say the same. It will cost you £200 for both front and back, but you dont get nuts for the front, If you want I know a guy who supplies the longer nuts and also longer locking nuts, these cost about £30 and its very very hard to find a supplier for the longer nuts.
Your speedo will be out by about 4 miles an hour too, doing 70mph on speedo is 74mph so be warned.
I bought my wheels with budget tyres on and found the grip to be real poor, so bought some dunlop sport maxx for rear and pirelli p zeros for front and its awesome now, but remember if your buying them with no tyres, your looking at another £230 for spacers and £500plus for new branded tyres, but there are plenty of part warns on ebay if you want to risk it.
For what ever reason, 19" snowflakes seam to the "in wheels" at the minute and are demanding big money, up to £850/900 with good tyres and in good nick. I declared them on my insurance, with a like for like replacement and my premium went up £300, not bad i supose because there £3000 at vauxhall new.
But dam they look sexy on an omega......
http://www.europerformance.co.uk/pages/guides/eibachsuspension/eibach-pro-spacers.mhtml