SATA-II isn't going to buy you any more performance over a regular SATA card, most RAID 5 cards are currently PCI, I do not believe there are any low cost PCI-E cards yet.
(** Please note the prices are outdated)
Highpoint Tech RocketRAID 1640 - 4 channel, PCI, Software Only RAID 5, NO cache memory
US$ 88 from NewEgg
Promise FastTrak S150 SX4 - 4 channel, PCI, Hardware Assisted RAID 5 Processor, Cache slot (user to install, takes standard PC100/133 SDRAM modules)
US$ 170 from NewEgg
LSI Logic MegaRAID SATA 150-4 - 4 channel, 64-bit PCI, Hardware RAID 5, 64MB Cache
US$ 247 from NewEgg
Adaptec SATA RAID 2410SA - 4 channel, 64-bit PCI, Hardware RAID 5, 64MB Cache
US$ 325 from NewEgg
3Ware Escalade 8506-4LP - 4 channel, 64-bit PCI, Hardware RAID 5, 64MB Cache
US$ 329 from NewEgg
Which card you choose in the end depends on your budget. But I highly recommend you NOT to get the Highpoint Tech card because its a pure Software RAID 5 implementation and the performance will depend on your CPU (which isn't too nice if you plan to use the machine other than a pure file server). The LSI Logic card right now has the best price/performance ratio IMO. If you have a spare stick of PC100/133 RAM around the Promise card is also OK as its even cheaper (I myself uses it and has a RAID 5 array of 3x160GB HDDs at the moment).
64-bit PCI cards are backwards compatible with your normal 32-bit 33MHz PCI slots on almost all motherboards (and it will operate at 32-bit mode).