Gigabit LAN is a good performance value to be had these days. Of course you can live without it, as we did for several years, but I personally wouldn't, when it's so cheap and potentially beneficial, esp. on a file server. You can add it via an adapter, and as long as the drivers are OK and you're not pushing performance very hard, I think this is a viable solution -- at low transfer speeds of around 30 MB/s, you're not likely to be bottlenecked by a PCI gigabit NIC, and this is still around 3x what you can do with 100 Mb/s ethernet.
A $25 RAID card isn't likely to do its own calculation -- it's going to do the calculations via the software drivers. Such cards are termed "fake RAID" in the Linux vernacular. I personally don't like the term "fake RAID", and am pretty happy with on-board nVIDIA RAID performance, but I think a claim of true hardware RAID for $25 needs some backing up.
I'd also like to hear about the performance implications of putting both the RAID card and a gigabit NIC on the PCI bus. If it can do the minimum -- 30 MB/s sustained, I'd call it a win, as it's still better than most if not all cheap pre-built NAS boxes, and matches basic desktop IDE + gigabit performance. As a point of reference, on-board nVIDIA RAID and gigabit can sustain > 70 MB/s in my experience.
OTOH, I think that a real backup solution can be more important in this context than raw performance. "RAID is not a backup." If you have significant amounts of personal data accumulating -- all your home movies, pictures, etc., which you can do with a file server, you should also think about how you're going to back all that up. RAID goes a good distance, but is not perfect (and often has its own problems). Even a periodic backup to a separate simple drive on the same computer for the critical stuff is a good start IMO. I understand that this is making the problem probably too complicated at the onset, but it's something worth thinking about in the longer run. A backup also gives you a lot of flexibility in changing your system around.
In the vein, aiming for lower amounts of total storage can be advantageous -- how are you going to backup a TB of stuff? Another TB server? It might be simpler to do 2x RAID 1 and separate drive that you use for the OS and a backup.
There are lots of options these days, and pretty much all of them are viable. I'm just sharing some thoughts and experience -- of course you should do what you think is best.