1GB DDR3 model of GT 240 is roughly the same as HD 4670/3850, so it's still better than GT 520. 1GB DDR5 model is roughly the same as GeForce 9600 GT or in some cases the 8800 GT.
Try asking for a GeForce GTS 250 if it is available, they probably have old stock. Or else go for XFX Radeon HD 5670.