What you should get depends on your current setup, which we know nothing about. We have no idea about your CPU, RAM, PSU, or even what resolution you play at. Had you bothered to actually give us info about your system, I/we could have given you advice.
I do know which cards are good for $150, perhaps it will help. You can certainly find an 8800GT for that amount, possibly even the 8800GTS. This is assuming of course you have a PCIe expansion slot. If your on AGP, your options are a lot less rosy. The best cards are the x1950pro, and the 3850. I have no idea how much the AGP versions of these cards are.
I'm gonna assume you have a PCIe slot on your mobo. Some good cards are (in addition to what he said) an ati hd 3870 or an nvidia 9800 gt. I've also seen a 9800gtx for $150, but that was an insane sale in early august. If you can find a 9800 gtx or an hd 4850 for $150, buy them.
I'd actually opt for an 8800 GT over a 9800 GT to save a few bucks. The only difference between them is the 9800 GT is 3-way SLI capable, but since your budget is at $150, it's probably safe to assume three cards are out of your price range.
It looks like the 4850 is just out of your price range, but if you could find one with a nice MIR, I'd opt for that. That card is one of the best bang-for-your-buck cards right now, so if you can muster up the cash, go for it!
It would be really helpful if you could provide us with your current setup!
At that, save money, grab a 9600gt and if the processor needs an upgrade do that, if any leftover, maybe more memory. Newegg had a 9600gt a day or 2 ago with free shipping for 85 bucks. Pretty decent deal if you ask me. And only a little slower than the 8800gt.
I read the topic header and my head screamed 4850. With good rebates you can hit $150 4850 easy, and its by far the best card in the range due to the architecture. Im sure a 9800 would run warhammer at 50 fps easy, but its not going to run it at 70+ as easily as a 4850 could.
If you just want to play the game, a 9600 gt or 4670 works as well. However, its a significant step down in terms of gpu power when you go from the gamer category to the high end media category, that contains these cards.
Price / performance go with the 4850. It supports 10.1 dx even, so it will last you longer.
Pretty much any card listed here would let you play warhammer fluidly. I play at 1600x1200 4x AA 8x AF @ 60+ FPS with a single 8800 GT (C2D E6750 @ 3.4 ghz, 2GB RAM, win xp pro) so as long as the rest of your hardware is somewhat recent, it will be playable.
A small word of warning though, the warhammer engine is quirky and some in-game settings just do not play well. For example, when I turn on both water effects, my framerate is cut by some 60% and loading/stuttering gets downright atrocious. Whichever card you go with, be sure you play around with the settings a bit once you get it, and don't freak out if things aren't quite right at the start.