The GTX + is a great deal at that price and performs pretty close to the 4870 (even identical IN certian games). Really I would go with the 9800gtx+ at that price, unless you game at 1900x1200 in which case the 4870 1GB is what I would get.
For the 35 dollar difference there is not enough performance difference for me to jump to the 512mb 4870
I would go for the 4870 512, The 9800GTX+ is about on par with the R4850, the 4870 is about as good as the Gtx 260 and for about 135$ i would snatch that little fella.
i think save 30$ and go for the 9800GTX+. it's a great performer if you play in 1280x1024 or 1680x1050. but, if you want more then buy the 4870 512mb.
Go for the most powerful card u can afford... and i think tat is the 4870 1GB because ur mobo has only 1 pcie slot...so it would make sense to get the single powerful card tat u can buy...
If you like AA. Pick the 4870 512MB.
If you play at higher than 1080p (1920x1080) then buy the Radeon 4870 1GB.
If you don't have any money or you don't want to use any AA then the 9800GTX will be enough.