First of all, welcome to the boards!
Second, assuming you're running a standard Cleric- IOW, there are no house rules stripping your PC of weapons or armor- you should be fine in combat. He just won't be "CoDzilla" with the War domain, etc.
What kind of PrCl would improve your PC's combat abilities, though, depends a little bit on what books you can use (as mentioned by Theroc) and the exact nature of the campaign. Your combat effectiveness may vary because some PrCls don't work well or work VERY well with certain campaign types.
By that I mean that a PC designed to combat Undead with particular fierceness will not be worth it in a campaign that sees very few Undead, but could be the MVPC in one that is crawling with them.
(I'm working without my books, so I can't be specific at this point.
List of prestige classes - Wikipedia, the free encyclopedia)
Your PC is a little low-level for the Dwarven Defender (DMG & SRD), but it is a nice PrCl to consider a couple of levels down the road. However, its not going to give you any spell levels- its a purely martial PrCl.
Another purely martial PrCl that will definitely boost your combat potential is the Kensai (CompWar), though you have to be lawful, and you'll need a +5BAB. The Justicar from the same book is also interesting.
The Warpriest (CompDiv) is another one to consider, as are the Holy Liberator and Church Inquisitor from the same book.
The Ordained Champion (CompChamp) is interesting, but as I recall, it may be limited to a certain faith.
The Shadowbane Inquisitor and Stalker (CompAdv) are quite flavorful.