The Jetway looks to be a pretty decent product - though it's a shame about the Realtek Ethernet controllers, which are not the best. Under FreeBSD (and hence pfSense), Intel controllers are usually regarded as the best, with the higher end Broadcom controllers often being not too far behind. The 32 bit PCI slot of the Jetway will act as a bandwidth limit on an external gigabit Ethernet card.
Soekris net 6501 boards have Intel Gigabit controllers, but they're relatively low end 82574 parts. These boards are also expensive, have a maximum of 2GByte RAM (a possible problem if you want to use RAM heavy packages such as Snort) and only have single core processors.
FreeBSD - and therefore pfSense - doesn't currently support RFC 4638, so is limited to a 1492 byte MTU on PPPoE. I've scoped out what needs doing to add this support, and hope to get round to implementing it and ultimately submitting patches, but I have no timescale for this work as I'm very busy at the moment.
PPPoE uses mpd for PPPoE, which does a limited amount of work in userland but the majority using the kernel's netgraph system. The use of netgraph means I can't recycle any of the work on implementing RFC 4638 in NetBSD and OpenBSD, which implement PPPoE very differently.
I'm 99% certain I will need a modest patch to ng_pppoe.c, as much of the PPPoE negotiation is done in the kernel without calling the userland callback. The necessary kernel patch is no more ambitious in scale than a patch I've previously had accepted for inclusion in FreeBSD. The userland support is pretty straightforward once the kernel support is in place.
If you want to use RFC 4638 for a 1500 byte MTU on BT Openreach FTTx, you need to select a network controller that supports jumbo frames of at least 1508 bytes. Many gigabit controllers do support jumbo frames, though the
FreeBSD 8.1 manpage for the re(4) driver isn't drawn on whether jumbo support is available for the 8111EVL controllers on the Jetway board you mention.