Help upgrading my computer!

I want to buy a new video card and I know i've got a slot for that but my computer is kinda too small.. would it be bad to just leave the case open?

Right now I have:

Pentium(R) Dual-Core E5300 2.60 GHz

Intel(R) G33/G31 Express Family Chipset

and 4GB of Ram

I want to be able to play new games so I want to get this video card

Newegg.com - SAPPHIRE VAPOR-X 100269VXL Radeon HD 4890 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFireX Support Video Card - Desktop Graphics / Video Cards

and this processor chip

Newegg.com - AMD Phenom II X4 945 Deneb 3.0GHz 4 x 512KB L2 Cache 6MB L3 Cache Socket AM3 125W Quad-Core Processor - Processors - Desktops

this'll sound dumb but could I do that or would I need to buy a new motherboard aswell? I bought the computer at bestbuy like this so I could improve upon it =], and I was hoping with this setup I could play the games I wanna play

So for sure the motherboard I have won't support that? Is there any way I can find out? Yeah I've been thinking about that... I have a tiny tiny case and even though it's cool-looking I think I'll need a new one! Any suggestions for that or just buy a cheap one that'll get the job done? And any suggestions for a motherboard that'll get the job done for not too high of a price?

So for sure the motherboard I have won't support that? Is there any way I can find out? Yeah I've been thinking about that... I have a tiny tiny case and even though it's cool-looking I think I'll need a new one! Any suggestions for that or just buy a cheap one that'll get the job done? And any suggestions for a motherboard that'll get the job done for not too high of a price?

cseru

You can't put an AMD chip in an Intel Motherboard. By the way you'll need a new PSU too