The first thing you need to do is identify the type of video card you have. Since you had an 8300 GS, chances are it is a PCI Express (PCIe) interfaced card. You can verify this if you still have the box for the card, look at your motherboard in comparison to this reference picture.

Once you verify that it is a PCIe card, since I don't want to be responsible for telling you to buy the wrong thing ;), then you have to look at what type of case you have. If you have a full sized desktop tower that is the standard width, then you are in luck. Your video card should look like this.

If you happen to have a low profile type of video card for a thin case, then you will have a bit of more work to do. Those types of video cards (low profile), look like this. Last but not least, there is a third type of video card style that is used for HTCP (home theater pcs) is a "half height" video card. Those look like this and are the harder ones to find. Those look like this and as you can see, are 1/2 the height of a regular card when inserted into your motherboard.

Once you know the interface for the card as well as the form factor for the card, you can then figure out the power options available. The top of the line cards nowadays take some form of: 2 x 6 pin power molex, 1 x 6 pin molex, or 1 x 8pin power molex. If you have an older computer, you simply won't have the power for these types of cards so you have to be careful. You will want to get a video card that does not require any additional power like your current 8300 GS is setup.

Since you already have an nVidia card, I'd assume you'd just keep with the brand. You will be looking at getting a 9XXX series of card that would be comparable or slightly better than your current card. That means anything between a 9300 to 9500 since those cards usually don't requite additional power and come in low profile and standard profile heights (half heights are more tricky and I'll assume you don't need one).

Basically when you look at a video card you will need to check:

1. The card's interface (you will mostly be finding PCIe 2.0 cards, which should be compatible with your PC assuming it currently uses PCIe)

2. The card's form factor. (If you have a low profile card right now, you need to get a low profile replacement. If not, either is fine.)

3. The card's power requirements. If you have a weak power supply or a power supply with no additional power connections for 6pin molexs you will want to find a card that does not require them.

And that's about it! Here's one suggestion of a card to check out via Newegg: XFX PVT94GZAFG GeForce 9400 GT 1GB 128-bit GDDR2 PCI Express 2.0 x16 HDCP Ready Video Card - Retail . That should be alright with your PC without knowing all the details. If your PC can't support that card, then you might be in trouble. [lol]

If you have any other questions or need more advice just ask!

About the power requirements, go to the various PSU calculators on the net and have them estimate your current consumption, then figure out if you have enough capacity for the new card. GPU power consumption estimates by manufacturers tend to be conservative but if you overtax your PSU you can get really cute effects like the PC turning off without any warning. Upgrading the PSU is a whole different story, apart from the extra expense you'd have to look at what form factor the PSU has - some models from certain vendors are non-standard, so you'd need to look at potentially getting a compatible upgrade.