It is difficult to suggest anything for best value of money, as everything depends on what you would like to do with your Amiga.
Mediator PCI is still available, see
www.shop.elbox.com . You can use Voodoo Banshee, Voodoo 3, 4 and 5 cards. As for networking cards you can use 10 MBit and 100 MBit ethernet cards. Look
here for a list of supported PCI cards.
As for IDE devices, PowerFlyer is still being produced, see the Elbox shop again, it is listed under FastATA. No accelerator for Amiga is being produced anymore, at least not in the form as they used to be made, so you will have to buy those second hand. You will also be able to find PPC boards second hand.
Some companies are working on new kind of accelerators, but I haven't got much info to tell on those, as I simply can not be bothered to read anything concerning them. I'm sure others will have some good information on them though, but one thing is sure, if you would like to go for one of those, you will have to wait a long time.