Yup x1900/7900 GTX are on par, but the x1900's use more power, and produce more heat, so if thats a problem then go for the GTX, if not then go for the x1900 as its cheaper, but they are on par as said.
Pointless upgrading to DX10 yet tho, £500 is only going to get you 1x game, the new Crysis one, and by the time the DX10 games start appearing, were going to have been through a fair few generations of cards, i can't see them flooding out, as the new Crysis one is the only one i know off, id wait until there is a fair few DX10 games out, and its worth buying a DX10 card, as by that time G80/R600 are going to be old hat, the cards we have now are going to run everything for a while yet.
Im sure there was a mention of about June/July for the G80, if so, i can't see the point in that as theres going to be nothing it can be used for DX10 wise, probably cost about £500, and will only be running the same exact stuff as the x1900's/x1800's, 7800/7900's, which are far cheaper.
If you get a DX10 card now, then when the DX10 stuff start appearing, your DX10 card you have now will probably not run em as well, so you'll be upgrading again probably to a faster one that is going to be able to handle the DX10 stuff properly as it came out around the same time.
Id just go for a x1900/7900, pointless going £300/£500, £500, then another £500 when the DX10 stuff is about, you may as well just go £300/£500, then the other £500 when the DX10 stuff is around, save yourself the price of the inbetween card.