Which graphics card to use on HP Elite m9040n to get better gaming performance

We would like to upgrade our graphics card to be able to get better gaming performance, in particular for GTA IV.

Currently we have NVIDIA Geforce 8400GS

We have the following PC:
Processor: Intel® Core™ 2 Quad Processor Q6600• 2.40 GHz, 2x4MB L2 Cache, 1066 MHz Front Side Bus; Platform: Intel® ViivTM Processor Technology; Chipset: Intel® G33 Express Chipset; Standard memory 3072MB PC2-5300 DDR2 SDRAM memory (2x1024MB 2x512MB) (We are expanding to 6GB); Memory type DDR2-SDRAM
Memory slots 4 DIMM (240-pin, DDR2) (occupied);System features
Video adapter NVIDIA GeForce 8400 GS graphics card; Video RAM Up to 1535MB Total Available Graphics Memory as allocated by Windows Vista;
Expansion slots: 1 PCI Slot (occupied) 2 PCI Express x1 slots (one available) 1 PCI Express x16 slot (occupied); The mother board is ASUS; Motherboard Name: IPIBL-LA
HP/Compaq motherboard name: Berkeley-GL8E.

I expect that when we replace the graphics card one of the expansion slots will become available, but I have no idea which one.

We are grateful for your help!

a 4850, 4870 or if you could wait and afford it, 5870.

An 8400GS is not powerful enough for...... almost any recent game.