PowerEdge 1750: setting up the raid

I would look at a system that runs SATA drives, SATA is a lot easier to find. You will want to look at the specs on the drives you purchase to see what the manufacturer states the sustained write speed is.

I can't say for sure on running XP on our servers. There are a couple of our lower end servers that are converted workstations. They have been validated for client operating systems, but most of our servers are not tested to work with any client OS. In theory you should be able to install XP on any system validated for server 2003, but there are subtle differences between the two OSs so you might have some issues.

You can find some really cheap 2950s for sale and in my opinion the 9th generation servers were one of the best lines we have ever come out with. You will see a noticeable performance difference between 9th gen servers and older servers. The tower version(2900) runs quieter and is a better choice to be used in a home if noise will be an issue. The 2900 tends to be more expensive because used tower servers are more sought after for home use.

Any rack server that is older than 10th generation is going to be very loud. 10th generation saw improvements with fan noise, and it has gotten continually better with later generations. If you are able to put the server/s in a garage or some room at the other end of your house then it shouldn't be an issue, but you likely won't be able to sleep or watch TV in the same room as a rack server. They are tolerable at idle fan speeds, but if the CPUs are highly utilized then the fans will max RPM and become extremely loud.

One other thing to consider is that you may be better off with a workstation or desktop computer. Is there a specific reason you are selecting a server to perform this video streaming function?

The 2950 has a SAS backplane.  SAS slots will accept SAS drives (with the uninterrupted connector) OR SATA drives (with the interrupted connector):

The break in the SATA drive connector lines up with a break in a SATA cable connector(s) (usually separate data/power cables), preventing you from attaching a SAS drive to a SATA cable/backplane connector. 

The SAS backplane/cable connector is open, so as to allow EITHER type of drive to be inserted.  SAS is "backward" compatible with SATA (backward being in quotes because it is more a "side" compatibility than really "backward").

The 2950 will take SAS or SATA.  They can even BOTH be used in the system, as long as they are not in the same array.  SAS disks are more expensive, because they are high-performance disks (robust enterprise-level commands, higher MTBF, and higher rotational speed).  SATA disks are lower-performance drives - a sacrifice made to allow for larger storage capacities.  If using a PERC controller, you should make sure the disks are enterprise-class disks if this is to house important data or services, as cheap consumer/desktop drives will not cut it.

"You seemed to suggest if using a PERC raid controller I'm locked into them anyway."

Not at all.  SAS disks are enterprise by default.  SATA comes in enterprise-class (generally more expensive, but programmed to work in RAID arrays for better performance and reliability) and consumer-class (these are your cheap desktop drives, which are not suitable for enterprise-level RAID arrays).  Consumer drives are not rated for 24x7 use, but even if you aren't using them 24x7, their commands are such that you may experience a number of different issues with them, ranging from benign entries in the controller's log to not even being recognized by the controller.

As for actual speed ... I've never really been one for benchmarking for certain speeds, but out of curiosity,

1.5GB file copied in 9 seconds (166MB/s) on the SSD in my workstation;

4.9GB file copied in 60 seconds (82MB/s) on a RAID 10 with four 15K SAS drives on PERC 6;

3.1GB file copied in 59 seconds (52MB/s) on a RAID 10 with four 15K SAS drives under a good load (terminal server) on PERC 6;

1GB file copied in 20 seconds (50MB/s) on a standalone (non-RAID) 15K SAS drive on a SAS 6/iR;

1GB file copied in 30 seconds (33MB/s) on a RAID 1 with 10K SAS drives on a SAS 6/iR;

3.7GB file copied in 110 seconds (33MB/s) on a standalone SATA drive in my workstation;

4.9GB file copied in 20 minutes (4MB/s) on a RAID 1 with SATA drives on a SAS 6/iR (wow is right);

So from this very unscientific survey of the servers convenient to me today, I think 35MB/s should not be hard to attain when using a decent controller (PERC) and enterprise-class disks.  If using SAS, two 15K disks in a RAID 1 may be sufficient.  If using SATA (which run at 7.2K), you would probably need at least 4, if not 6, in a RAID 10 to sustain those speeds.  You said your storage needs are "not that great", so size depends on what you need ... 73GB/15K SAS drives are pretty reasonably priced - 4xRAID10=138GB of usable space to the OS.  Of course, this can all depend on the software, the files, and how it is communicated to the controlller that might affect actual speeds.

Negligible.  The biggest difference between the PERC 5 and PERC 6 (newer) is that the PERC 6 supports RAID 6 and 60. The PERC 6 came out mid-life to the 2950, so older 2950's would have shipped with a PERC 5, newer ones with a PERC 6 ... both will work.