NO, the 4870 isn't even in the same performance category as the 9800GTX+, if your comparing the HD4850, then it'd be ok to compare, but the HD4870 competes with the GTX260 (both the 192 & 216 Stream Processor models) & GTX280 cards (4870 is close to the GTX280, but is about on par with the GTX260 216 core version, with the 192 core 260 trailing behind that. Geforce 9800GX2 is also up in this performance range as well, along with the HD4870x2 also, the latter being the single fastest most powerful card you can buy right now).
But if you were comparing the 9800GTX+ to the HD4850, then it'd be hard to say, most will say the 4850 will be the best deal, as its usually a bit cheaper, but they both perform close to the same, maybe with a bit of a nod to the 4850.