In your first post you mention 'major HP increases' and then you say in subsequent ones you just want a 'little' more increase! if you are planning on getting a 540i or an M5 why not just save the cash you were going to spend on the 528i and put it into the funds for purchasing one of those faster variants ?!
If you do want a little more on the 528i anyway - look into the Vanos seals as already mentioned, if they've already been done and your engine is in a good healthy state, get a good quality re-map. You'll perhaps gain 5-8 genuine HP but the torque delivery is much improved and 'feels' as though you've gained more. Flat spots and hesitation in the rev range are removed and if you wish, you can increase the rev limiter by 500 rpm. There are many on this forum who'll say it's a 'waste of time' - but they generally are 'only' looking at HP increases and NOT the enhanced Torque delivery.