Sad thing is I made this topic before but I searched for it and I couldn't find it.. Weird.

Well my computer is an HP Pavillion Slimeline s3242x
http://h10025.www1.hp.com/ewfr.....ct=3628086
Thats my computer exactly.

It has a PCI Express x16 slot but the thing is, its a SLIMLINE. So im looking for a video card for a 100$ give or take 20$ and I don't know where to look. Can you guys help me out?

Use the Power Search at newegg and set the "Form Factor" to "Low Profile" and the Interface to PCI-E x16 and PCI-E 2.0 x16. Here are the results I get, assuming this URL can do that:
http://www.newegg.com/Product/.....696%3A9641

There is also Form Factor of "Low profile ready", but I'm not sure what that means.

Jesse, there are two things that you should verify before getting a video card. First make sure that it will fit in the case. KFA says the height of the board is 80mm. Second, you need to check the power requirements of the CPU and the graphics card to make sure that they don't exceed the amount of power available from the system's Power Supply Unit (PSU). What is the power spec on the PSU? I'm not finding it at HP.

If the PSU spec is not in any documentation that you have or on HP's website, the only way would be to examine it. The specs are written on the PSU, usually inside the case. Yes, that means opening the case which you will have to do to add a graphics adapter anyway. While you are in there, you could check to see if there is 80mm of clearance for the card and room for the slot cover with the connectors on it.

Ouch! That is a very low capacity power supply. I guess that I shouldn't be surprised. A slimline system is designed to be low power, low noise, low profile, perhaps primarily, low cost. If you read the user comments at newegg about the KFA2 GeForce 8400 card, they say that Nvidia recommends a 350W power supply on the box. I have not been able to confirm this, but note the comments from 2 users who had 280W and 275W PSUs and said they got it working. They knew that they were below the recommended power.

Another power draw of concern is the CPU which HP lists as an Athlon64 X2 with a TDP of 65W. That's quite a bit of power gone right there, so any graphics adapter would have to be a very low power design to work in this system. I'm not sure there is such a card, but I'm pretty certain that the KFA2 8400 is not going to work.