I just bought a Proliance Heatbuster Extreme from RockAuto.com for $170 -5% discount + shipping. It's a 3-core radiator with aluminum tanks. It cools much better than the stock setup and I would not hesitate to recommend it. I would also recommend replacing your hoses at this point. I changed only the upper hose, and now I'm going to have to go back and replace the lower hose and the overflow hose.
If you want to order from RockAuto.com and you need a code for the 5% discount, let me know and I'd be happy to post my most recent code.
Some, er, fine mechanic had overtightened the upper radiator hose clamp on my old one, and had cracked the plastic nipple.
My XJ is a '96 4.0 automatic, BTW.