I currently have a 1.33GHZ Thunderbird, 512MB DDR, with a GeForce2 MX.
 I want to upgrade my video card, preferably to something with video
in/out so I can watch movies on my tv and use my comp as a pvr and to
increase frame rates.  My problem is that my motherboard (MSI K7T266
Pro) only has a 4x AGP slot.

If I upgrade to a GeForce4 Ti, will an 8x card work in a 4x slot? 
Will I notice any performance benifits?  If so, what is a good card
for the price, preferably $150-175 or so, no 9700s needed here.

Hello! Hope the tip below in the Comments helped, but I decided to go
ahead and do the stuff and provide your answer.

An 8x Geforce4 Ti video card will run on 4x, but as 4x. It's like RAM,
you can use higher RAM on a lower-speed slot (such as PC150 in a PC133
slot, but not the other way around). The TV-out will work. It is
doubtful if there will be any bonus benefits from the high performance
card. I see you have a DDR motherboard, so I hope you know that
alternative video cards should also be DDR.

Allexperts.com by About.com - question about a video card
http://www.allexperts.com/previousqv.asp?QuestionID=2706931

Alternative card:
I think only ATI can rival the GeForce in quality. The 9700 sure is
costly, so I wonder if this will do:

ATI Radeon 9000 Pro 128MB = $149 highest price, $101 lowest
http://tomshardware.bizrate.com/marketplace/product_info/overview/index__cat_id--405,prod_id--7349133,de_id--300,name--ATI%20Technologies%20Radeon%209000%20PRO%20128MB%20AGP%20Dual%20W-VGA-DVI-I.html

Specifications for the 9000:
http://mirror.ati.com/products/pc/radeon9000pro/specs.html

For a less expensive Ti, see if Inno3d or Pine cards are available in
your area.

Search strategy on Google:
4x video card run 8x agp

Other strategy:
Browsing on Tom's Hardware Guide
http://www.tomshardware.com

Try also Basic Hardware, a hardware review site, for recommendations:
http://www.basichardware.com

Hope this helped.

Let me offer a tip.
You may want to consult Tom's Hardware Guide in looking for
alternative video cards.

http://www.tomshardware.com

You can use the "Search Prices" bar at the left to look for other
video cards for sale. Try "4x" and select the "Audio/Video" Category.