The first question would be does your HTPC have a GB NIC? If not, this whole discussion is pointless.
From, "SATA II (revision 2.x) interface, formally known as SATA 3Gb/s, is a second generation SATA interface running at 3.0 Gb/s. The bandwidth throughput, which is supported by the interface, is up to 300MB/s"
So, even if your drives took a 50% performance hit in RAID5 they would still be faster than the speed supported by your current switch. While the drives will be the bottleneck if you use a GB switch you will see a noticeable performance increase over the 10/100 switch when performing actions with large amounts data.