Check these out first:
http://www23.tomshardware.com/graphics_2007.htmlhttp://www23.tomshardware.com/graphics_2006.htmlhttp://www23.tomshardware.com/graphics2005.htmlThese will give you an idea of performance in comparison to other cards. If you want to check out older cards, check 2005 and 2006, otherwise look for a low / mid-range 2007 card.
I can't say much about Nvidia having never owned one (or a real one at least. MX Series don't really count!)
However, I have owned a lot of ATI cards, one of which was an X850XT PE. This is an example of quite an old card which really did hold up well till recently, and since upping my resolution. I think it was a similar story for the Nvidia 7800s, and these are really cheap cards.
Avoid the X1300 through X1600 cards, and the X1800 upwards are still great but are too pricey for your budget. I'm still running a X1900XTX and it can still kick ass!
Maybe have a look round for the X800 / X850 range, and look out for the crap on the end. The XL and Pro suck, the XT and XT PE are much better versions of these cards. But as I said, with your budget try and get second hand - you'll get a lot more for your money.
On the other hand, you could go for a new 2007 / 2008 DX10 model, but you'll pay the price. They may be new and HD and all that jizzle, but they aren't too fast. You'll probably pay less for a brand new 8600 than a X800 which is a four year old card, and will beat the 8 series hands down.
Personally, if I really couldn't scrape any more cash together now, and depending on how desperate I was to do some serious gaming, I would find a dirt cheap, any old piece-o-shit PCI-E card for about ?10, slap it into the new build, and then XXX amount of time later, upgrade it. That XXX amount of time will be hell, what with upgrading the rest and not reaping the benefits, but afterwards you'll be grateful!