Rather than spending a ton on a new GFX Card, why don't you simply buy a new motherboard? Motherboards are tons tons cheaper than a Graphics Card is.

Reasons i think you should get a new motherboard:
1) Cheaper
2) PCI isn't as fast as AGP
3) Is it really a good motherboard if it isn't an AGP slot? I don't think so.

Also if your PC is a package type (One which you buy with everything already in it), then the chances are they would have cut back on a lot of hardware to save themselves money and increase the profit margin. If they don't mention it in the specs, it's going to be cut back and suck a ton.

A package PC normally comes with:
1) A crap motherboard.
2) A crap and inefficient heatsink.
3) No Case fan at all.
4) Low watt PSU, which results in computer instability if you expand your PC with powerful (but power consuming) hardware.
5) The cheaper version of anything they promised.
(EG: If they say a GeForce 4 GFX Card, it's probably a GeForce 4 16mb Graphics Card, which is pure crap and junk. It's even cheaper than the GeForce 2 128mb Graphics Card.)
6) They limit the computer as much as possible so you can't expand, and instead charge you to 'upgrade' your computer, as well as void any warrenty you have for simply opening your computer.

That is why i NEVER buy package PC's. Companies Rip you off. They not only charge 200% more than what it would of cost if it was as good as they said it was (I worked out i was ripped off by over 1,200 pounds by Tiny.), but they also rip you off by giving you cheaper hardware which is allowed as long as it isn't mentioned in the spec. Always make your own PC and you could save yourself between 500 to 1,000 in costs, and you also get 5 times better quality hardware, because you would of picked it yourself and not cut corners like the companies do. Also once you're on a package PC, you're Stuck in their money ripping ideas. For instance, if you want to replace a dodgy part and you know what it is, you can't do it yourself. You have to ring the company up you got it from, and send it to them to find and sort out. Which they charge you to do, or if they don't charge you, they instead charge you for extended warrenty, and then they pretend to fail to find the problem 5 times and then threaten to charge you 60 odd if you send it back to them again.

The lifetime of their hardware is also exceptionally short in comparison to ones you would buy yourself. That's because when they put the computers together, they don't use the correct procedure. Hell they even stack hardware on top of each other and let the weight and static destroy them.

Also you can't simply upgrade a part with the newest thing, because you void your warrenty as soon as you open your case >_<
Even if you could open your case, they limit you as much as possible so you can't expand. Like getting a motherboard with No AGP and even if it has one, they limit as many PCI slots you have as possible and try to melt the IDE Cables down, and place tons and tons of Cable Ties to stop you from taking hardware out. Also their customised cases are normally so chunked up together, that you can't do custom stuff to the case, or expand the IDE Slots at the front, or change the Disk Drive because they rip the cover off theirs to get it into their so called customised case.

So instead, they charge you to 'upgrade' your hardware. They charge you 200% more than it would originally cost for the upgraded parts, and even then they STILL give you the cheaper versions of those parts with a very small lifespan.

When you buy stuff yourself, you get a warrenty on each part you buy. Which means if you can figure out what part it is, you can get that exact part replaced instantly instead of sending off the entire PC and being charged for it and crap. Also you can't void your warrenty for opening your case, because the case was made specifically for people who are going to be building the PC up all the time, and also the parts you buy won't be as limited as possible or have tons of crappy cable ties everywhere, becaue your the one building it here. Even the cases you would buy to build your own PC will have expandability in mind because they would of had a ton of spare IDE Slots at the front for more CD-ROMs or whatever you wanted.

Anyway... i would suggest buying a new motherboard and forgetting the warrenty crap the company give you. If a part goes wrong, just buy the part separatly and change it yourself. Never use company crap.

Edit:
Besides, it's probably a ton cheaper to buy a new motherboard than a new Graphics Card. The motherboards cost tons and tons less than a Graphics Card does.