the x800 i belive is faster regardless... memory doesnt matter any bit. if you have a lot of it, on a slow card, it wont matter one bit.
more memory just means you can likly run at higher resolutions with better quality textures... ie doom3 on ultra mode... but then there is exceptions, like the actuall speed of the core and memory. if its to slow then it wont matter.
512 on a slow card or 256 on a faster card (tho how much faster, ehhhh) its not going to make a differnce really, the faster card is still going to be better. tho the x1xxx serise of cards does offer alot more features, like shader model3 support.
i have found x800 cards for cheaper than 350$ you just have to do some shopping. i bought myne straight from ati, referbished for 280$ i belive. and that was a while ago, and since then i have seen deals for it around 300$ give or take just a few dollers. but myne was an X800XT PE, the top of the line before the x850(and up).
dont factor out nvidia prices, im not sure what they offer in price since i havent looked lately. but if you wait a while, when the new chips release, prices will go down. this will happen around october give or take a month.
annother note, i would just start a new computer from scratch.... now this would take time, but would be great because of all the upgrading being done, like AMD's AM2 motherboards with pci-express and pci-express videocards. when amd gets a new chip, called AM3, you will be able to use that chip in your AM2 motherboard, they are pin compatible and should only need a bios update - however they wont be as fast (because hyper transport2 is what those boards use as a bus technology, and AM3 is designed with hypertransport3, but will work with the older technology - just an update).
new DDR2, even tho i dont like it one bit, is out and its here to stay, and getting faster and faster every week it seems.
and the final point of upgrading is to be vista capeable. now... this you would have to wait a little while longer. basially vista requires 1 or 2 gigs of memory, working just ok with 512mb's minus the flashy 3d display... the faster and higher bandwidth the memory the better ie ddr2.
faster processor = better for vista, and more cores will benifit - intel is comming out with a new dual core duo to rival amd, and its looking very practical, i have been an AMD fan because it is cost effective and the performance leader, now intel is looking to make a change to their buisness and it MIGHT be for the better (highlight MIGHT, its not out yet).
pci-express is probably going to be needed, i belive with directX 10 in vista, its probably going to be some what manditory, i dont see much legacy stuff thats going to benifit. so a new video card is needed, this wait is off by 3 to 6 months (seeing that we have to wait almost 6 months for vista to release (hey mabe longer)... directx 10 is vista only... and a great gaming platform... vista's only real point to buy is for gaming, otherwise no one would buy it

its winXP with directx10 and then bloat on top of it all (they want us to spend money on hardware, which is good, but... you get the point, a bad for every good).
why waste the money now if we will just have to do it later... save the money now and collect interest for a better system later (thats why they have a bank).
anyways... here are some prices...
x800XT all in wonder... $243.69
http://www.monarchcomputer.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=M&Product_Code=190667&AFFIL=pricewatch&NR=1ATI All-In-Wonder X800 XT 100-714200 Radeon X800XT 256MB 256-bit GDDR3 VIVO AGP 4X/8X Video Card - Retail
$269.99
http://www.newegg.com/Product/Product.asp?Item=N82E16814102492&CMP=OTC-pr1c3watch&ATT=14-102-492Connect3D Radeon X850 XT Platinum Price - $259.69
http://www.pricewatch.com/public/info.aspx?i=37&z=1674&ro=2&aid=5670269VisionTek XTASY Radeon X850 XT $155
http://www.pricewatch.com/public/info.aspx?i=37&z=1674&ro=1&aid=11740632Connect3D Radeon X850 Pro 256MB AGP $210.99
http://www.pricewatch.com/public/info.aspx?i=37&z=1674&ro=1&aid=8291873so those are all under 300$ and... they are all pritty fast, THO do your research first!!!!!!!! an XT vs an XT PE or 800xt pe vs x850XT ... you have to know the differnce, tho one is faster in specs on paper, in real world performance it may DIFFER GREATLY... do the review thing, and SEARCH! dont get ripped off, if your going to pay ABOUT the same amount of money for a card then choose the right one.