Can someone answer some basic PC hardware questions for me please?

I have a Dell Inspiron 530S desktop. It's old, but it's what I got. I'm trying to upgrade it a little bit if it's possible to make some games run smoother for my son.

It has an Intel Core 2 Duo CPU
E6550 @ 2.33Ghz
2.00 GB of RAM

The video card is an ATI Radeon HD 2400 PRO

My questions are:
- Slots #4 and #6 are open, can I just pop RAM into them? Will that help?

-What would be a good video card that would work with this PC? Is it as simple as unplugging the card that's in slot #24 and putting in a better one?

I really, really appreciate any input.

This is the schematic of the internals:


1 processor socket (CPU)
2 processor fan connector (CPU_FAN)
3 memory module connectors (DIMM_1)
4 memory module connectors (DIMM_2)
5 memory module connectors (DIMM_3)
6 memory module connectors (DIMM_4)
7 main power connector (ATX_POWER)
8 Floppy Drive Connector
9 battery socket
10 serial ATA drive connectors (SATA0)
11 CMOS jumper (CLEAR CMOS)
12 password jumper (CLEAR_PW)
13 serial ATA drive connectors (SATA1)
14 serial ATA drive connectors (SATA4)
15 front panel connector (F_PANEL)
16 front USB connector (F_USB1)
17 serial ATA drive connectors (SATA5)
18 front USB connector (F_USB2)
19 front FlexBay connector (F_USB3)
20 front audio (F_AUDIO)
21 PCI connector (PCI2)
22 PCI connector (PCI1)
23 PCI Express x1 connector (PCIE_X1)
24 PCI Express x16 connector (PCIE_X16)
25 audio connectors
26 2 USB and 1 LAN connector
27 2 USB connectors
28 chassis fan connector (CHASSIS_FAN)
29 video connector (VGA)
30 power for cpu (ATX_CPU)