Finished my $300 system, how to upgrade over the next 6 months?

I finally got around to finish my system that is meant to serve two needs: 1. Networked storage for the five laptops in my apartment and 2. HTPC replacement for my Macbook Pro currently running boxee. Here it is (prices are after MIRs):

Intel E5200 - $51.29
eVGA nForce 730i w/ nVIDIA 9300 onboard GPU - $67
Antec Three Hundred - $60
Corsair CMPSU-400CX 400W - $40
Patriot Extreme 4GB DDR2-800 RAM - $24
Hitachi 1TB HD - $70
Shipping & Tax - $20
Total = $332.29 (within about $5 as I rounded some prices)

I went with a pricier case than you'd probably expect in a build at this budget (could've finished it at around $275) because this thing is going to be in the middle of my living room, so I want it to look good. Also, the Three Hundred has a great reputation and tons of expansion bays which are key given my intended uses.

My question is, simply, what would you do to this computer over the next 6 months at $100/month? I'm not necessarily saying I WILL spend that, as it will most likely be unnecessary, but its a number to work with.

Right now I'm mainly planning two things:
1. A DVD Burner - $25 - This is obvious, and I intended to include it in my original build but went over budget.

2. Storage/RAID Array - I figure I can add in a 1TB drive every month at around $70-80 depending on deals, until I'm at a comfortable amount of space (3TB useable.) I've been doing research on RAID arrays as well and though not quite at the point of knowing 100% how to implement one into my system, several setups seem desirable for my situation (RAID 5 and RAID 10 in particular.)

I may also eventually transfer the components into a nice HTPC case and use the Antec Three Hundred for a new project. Does anything else seem worthwhile?

While I'm posting about it I may as well throw this in: what OS would you run on this? I'm thinking Linux because I'd like to run boxee and NAS type setups seem easy, but I'm still considering the fallback of Vista to avoid any compatability related hassle.

Thanks.

A BD Rom drive, as you say more storage, maybe a xonar soundcard in the long term, maybe consider a hardware RAID card if there's several machines hammering the storage.