I want to upgrade it, but i really don't know how to choose a nice card for my system:
AMD Athlon(tm) 64 X2 Dual Core Processor 4800+
4GB DDR2 800MHz
ATI Radeon HD 2400 Series (256MB, PCIe 1.00 x16, SM 4.0)
Microsoft Windows Vista Ultimate 6.00.6001 (Service Pack 1)
To be honest with ya, since it's a slimline system, you may not be able to get too much better than that card. Since most cards with reasonable power don't come in a Low-Profile form.
The best [low-profile] card on newegg would probably be an 8500GT. But that isn't even that great.