Thanks for the update and congratulations on the success! Now I'm wondering if my old PSU would power mine. It only had 16 A on the single +12v rail. I'm not going to bother with it.
http://newbling.net/BenchmarkImages/HD3850/ The card only uses just over 60W, you need a good PSU only because they seem like to have stable 12V instantly on boot. If your PSU more than 4 years old, a new PSU is not really a waste as you could reuse it in 2 years. Also for P4's you need a good quality old motherboard with an intel chipset, hyperthreading and DDR Dual channel or DDR2 to make it worth it. If you are not sure what condition you motherboard is in or have a non-standard MB then it is a bit of a punt.
Good choice, 2600XT is actually worse than the X1950 Pro (although I can't recall how much but from all I have read it is), so it's a good thing you got the X1950 Pro in the end, power supply would need updating a bit though, I had a 450W power supply with 18A on the +12v and it ended up dying, just to let you know.
I'm really late but oh well... No one should buy the 7600GT with DDR2, that is just total crap. For that you might as well go with a 2600XT for less than $120. But I agree, no point in spending a lot of money for potential you might not even be able to tap into due to being held back by your CPU's limitations. I probably should have looked more into the X1950 but I didn't really bother I don't think there's any doubt that it's superior in processing power but I read too much about the heat (though I guess not much hotter than my 6600GT) and power consumption being high that I would have went with a 3850 instead even if it was severely bottlenecked by my CPU. Last night I finally installed the Sapphire 2600XT 512MB (I know it's pretty much pointless with a 128-bit memory bus) I received over a week ago and haven't had any problems yet as many others seemed to have, I just uninstalled everything NVidia related, ran Driver Cleaner Pro in safe mode to get the rest, installed the card and then 8.3 hotfix drivers. I've only played two games (Oblivion and HL2) and I have no complaints, it's a nice improvement over my 128MB XFX 6600GT with better frame rates and higher detail in Oblivion and resolution in HL2 along with AA and AF at still playable frame rates, averaged around 45-50 FPS with AA at 8x and 77 with AA off, good enough for me since my resolution only goes up to 1280x1024 (65Hz) anyway.. I'm going to try 1280x1024 in Oblivion since I haven't yet but my frame rates were better/more stable and at the worst, even with the 6600GT with graphical options that were reduced or turned off before. Hopefully better drivers will come out soon and improve things more. My card's hottest that I've measured so far is 62C at full load, though that was only for a few seconds, it fell quickly when less GPU resources were needed, fan has yet to run at full speed also. It's idling now at 47/48C which is around the same as my old 6600GT but much quieter of course the fan not yet running at 100% might be a good reason for that. I've also noticed that the memory and GPU are slightly under clocked for some reason with the GPU core at 796.5MHz instead of 800Mhz and the memory at 693/1386MHz instead of 700/1400MHz.
Check this out: http://www.overclockers.co.uk/productlist.php?groupid=701&catid=56&sortby=nameAsc&subid=403&mfrid=
Well if you take my opinion then i will go for Saphire HD 2600 XT 512MB because for you and me this card work fine because we got a p4 system with a not much powerfull processor power so if you want performance and quality and then this card will be suitable for your system as it is working in my system its running like a charm for me i can play games with good frame rates.But last its your desicion to make. Every body here will say to you to buy the 3850 and i will say it to to buy this high end card but it is worthless if you have a p4 if you really want to play games and do high end stuff then buy a new rig that can support the 3850 then again if you change your rig then buy the 3870 Which is my personal favourite. Choices are yours and yours alone.
Go for the X1950 Pro, I had it in an AMD Athlon 64 3000+ system and it was fine, the P4 should be a bit faster than that.
The AMD gaming performance is superior to any Pentium 4, even an Athlon 64 2800+ can outperform a Pentium 4 3.0GHz in most games with the same rig configuration.