I agree with you, the next best step would be the center channel. What is your budget? How big is your room? What frequency do you crossover your mains to your sub?
I've ordered parts for this design of a center channel, it won't be built for about another month though so I can't tell you how it sounds. I am excited about it though.
http://www.htguide.com/forum/showthread.php4?t=22393
There are also smaller and less expensive designs at htguide.com
Supposedly Zaph Audio's ZDT3C center channel is a good one
http://www.zaphaudio.com/ZDT3.5.html
Also the Loki kit that Isibirian recommended would be an excellent choice for a beginner project. I would recommend using the suggested ported dimensions on the Loki page.
http://www.madisound.com/catalog/product_info.php?products_id=1696
I bring these builds up because they are all designs I debated over before making my decision on what to build.
Odds are if you are doing a well regarded, documented DIY design it will end up sounding very good. Especially in comparison to a HTIB center channel
Just try something and enjoy it.