With a decent quality 400 w power supply you have several good options. A 9600GT, a HD3870 or a HD4670. Other options would include a HD4830 and a 9800GT, but they'd be pushing your budget. I'd add 1GB of memory even with XP.
HD 4670 $70 after rebate- best choice if your power supply is also 4 years old and/or not a top quality unit. Decent performance for gaming up to 1440x900 resolution, good cooling, make sure it will fit in your system.http://www.newegg.com/Product/Product.aspx?Item=N82E16814161252
HD3870 -$65 after rebate A bit better performance then the HD4670 It also runs hotter, uses more power, but it is a single slot cooling solution. Up to 1680X1050http://www.newegg.com/Product/Product.aspx?Item=N82E16814102752
9600GT - $65 after rebate - Basically the same as the HD3870 in it's use of power and it's performance. 2 slot card with decent cooling.http://www.newegg.com/Product/Product.aspx?Item=N82E16814500077
HD4830 - $80 after rebate - The most powerful and the most power hungry of the cards listed. I'd only use this one with a good condition/quality 400w power supply, very good performance up to 1680X105 resolution.http://www.newegg.com/Product/Product.aspx?Item=N82E16814131129
All the cards listed would do a pretty good job with WoW at 1440x900 (19inch monitor), but i'd also fit that extra RAM into your budget.