No, you're not wrong. The higher the number the "faster" it is. The first number is the generation though, as you might have guessed, 7xxx, is the seventh generation of GeForce cards. And yeah, you said 7800 and linked to a 7600.
Let's see ... I can give you some links though. $199, total, or less for a good, reliable card, with lots of warranty.
You seem to be interested in GeForce cards, so I'll stick to that. Feel free to browse these sites yourself, of course.
The only issue is, you want AGP, since it's being phased out, it's actually getting to be more expensive than PCI-E cards.
BFG GF7600 GS 512MB ($149.99 + $5.64)
I'm not vouching for BFG, I don't know much about them. I don't see any warranty information either. You might want to look into that and if you're ever really concerned with that you can purchase newegg warranty. However, they do charge for any sort of replacement (15%). And the warranty it self is $30 to $50 for 1 or 2 years.
On the up side, that's a 512MB video card for less than $200. Yes, this should be able to play anything you throw at it that is currently out.
XFX GF7600 GS 512MB ($153.99 + $5.64)
All of the above comments apply to this card as well.
But, this card seems to have HD support. Not sure if the other card does (it says it has HDTV out, but says nothing about hardware support)
http://www.zipzoomfly.com/jsp/ProductDetail.jsp?ProductCode=326049
Same as the first card, just cheaper (free shipping. Also, zipzoomfly has the option of $1.99 shipping, if you want it there pretty quick, select that)
That seems to be it. I really wouldn't suggest getting a 256MB card, because, it wont be much cheaper and this should be the last AGP card you buy, so go out with a bang.
As long as the hardware lasts on these cards, there should be no issue when it comes to games (until DX10 games start coming out, of course)
There was a MSI 7600GS also, and while MSI has nice warranties (usually 3 years) the card has less features (no HD support). The XFX seems like the best "deal" here. Again, the options are limited because of the memory size (which I suggest being very large so the card has longevity) and the slot type.
And, lastly, put that damn link into an url tag so the page doesn't have horizontal scroll. Please.
Blame it on Microsoft, God does.