Im fitting 650's to mine next weekend, so my STI Pinks (somewhere between 530 and 565, depending who you ask) will not go past 380bhp.
Bear in mind the bigger you go, the more erratic you idle may become.
550's would be the obvious choice, with adaptors to fit your car, or you could go for 740's at the same price and leave future modding open.
If you buy from the UK, its quite expensive, if you buy from the states its cheaper, but you have to know what you are doing, when it comes to duty costs.