Dont run WinXP as performance will suck. But since you are looking to go linux, you wont have to worry about that.
The advantages RAID 5 gives you is that one dirve can fail and it will still keep on going, without loss of data or downtime until you go swap in a new drive and tell it to restore (which it do without any down time).
Than rsync from that, off site in case something happens that makes all 3 fubar (power surge, flood, pebkac (happens to me more than powersurges do) etc).
Actually, with RAID 5 if you have 3x70GB drives, you have 140GB of useable storage. If you have RAID 1 with 2x70, you only have 70GB of storage, and you are using the 3rd for backup. Even if you do 3x70GB of RAID 1, its only 70GB of storage. RAID 5 would be giving you more room. The forumal is:
The 170 is a single channel (only 15 drives) card. It uses a 32 bit PCI slot. A 32bit PCI slot (which runs at 33MHz) gives you 133MBps of maximum bandwidth.
If you want to make use of the 352's 64 bit PCI slot (which runs at 33MHz also, not 66MHz like some 64bit PCI), you will need a motherboard which has a 64 bit 33MHz PCI slot. A 64 bit slot gives you 266MBps of bandwidth. The 352 also gives you a second channel (15 devices per chanel lets you use 30 devices on this card).
A U160 channel gives you, yes you guessed it, 160 MBps to play with.
Of course all these numbers are just the calculated theoretical maximum throughput. Bus overhead, etc etc will prevent you from actually obtaining these numbers.
What harddrives are you looking to use? If you aren't looking for the fastest things and don't plan on hitting the 32bit PCI limitation, the 170 should be fine, but if you do want more throughput, the 352 would do better. Of course, if you don't have a mobo with a 64bit PCI slot, the decision is made for you already.
Now of course you need to somehow fit this all into a 2U case some how as well
You aren't looking to build a new server to colo phpBB, are you? (Don't mind me, I'm just curious and noticed your past few posts)