Yeah, HY27's got it right, althoguh to add that the RSX also only has half the bandwidth of the GT at 128bit, and of course it's 'shared' memory. And while the Cell is helpful, it still suffers many of the same limitations of the GF7 series and relying on even the CELL to make up fo them is still prohibitive because of the added latency per cycle to virtualize operations that would perform much faster if they were supported in hardware, it's like being able to do HDR+AA by sending the image back through the ROPs a second time, it's possible, but the performance penalty makes it not practical.
Homerdog's got it close, with the Xenos having about the equivalent strength of a X1950Pro-X1950XT but with some extra little bits that would be similar to the HD2900.
While the RSX is the same architecture as a desktop part the Xenos is not, and add to that the added variable of eDRAM and there's really nothing similar to it.
Features and Performance wise though I'd say it's like an ATi X19xx series with some bonuses, but it definitely falls short of the GF8800/HD2K/3K series cards for both features and performance.