Standalone NAS is nice, but do any of them do hardware RAID-5? RAID-0 is fast, but lose a drive and lose your data. RAID-1 is nice, you can lose a drive and keep all of your data...but it's not a very economical use of your drives. RAID-5 is the way to go, you can RAID 8 or 12 drives and only lose a single drive's capacity due to redundancy. I haven't seen any small NAS boxes that will do it though.
Instead, I'd recommend searching eBay for 3ware products, specifically 7506-8 cards. These 64-bit, 66MHz PCI PATA cards will do RAID-5 on up to 8 PATA drives, and can be had for around $150. Get a cheap case with a lot of bays, a cheap mobo/CPU/RAM and you're in business. It doesn't even have to be a very fast system...the hardware RAID card handles everything, the CPU just needs to get the data from the RAID card to the Gigabit NIC.
I recently put together a system like this. Just to make sure I was getting the absolute best possible performance, I bought an Intel mobo with dual 933MHz CPUs and two 64-bit, 66MHz PCI slots...total cost, about $80. Add in the 64-bit, 66MHz 3ware RAID card (7506-8) that I paid $120 for, and the total cost is about $200. (I already had the case/PS/RAM/Gigabit NIC, so those were free to me.) This is a kick-ass RAID-5 NAS solution. I run it under Windows, but it could easily run under Linux with the 3ware drivers. It's blazingly fast, cheap and reliable. Also, it's upgradable to larger drives in the future. There's also a 7506-12 that will support 12 drives, but the price on those is too high right now to get a good bang for your buck. It's a better deal to get a couple of -8 cards in most cases.
By the way, you can plug a 64-bit, 66MHz RAID controller into your PC. The 66MHz card will auto-adjust down to 33MHz, and the extra 32-bits of the PCI edge card connector are designed to hand past the end of your 32-bit slot and just run with slightly inferior performance.
Mmmmm...RAID-5 goodness is almost as good as dark, dark chocolate.