IDing bottlenecks

[Oberon]

G'day all

I'm looking to upgrade this machine a bit to a point where I can run SHIII with GWX Gold smoothly (at the moment it lags like crazy in 3D external and periscope views), and if possible also run Empire: Total War and ARMA1.

My ultimate dream would be to have it run ARMAII but I fear that's well beyond the VERY limited budget I have.

So, I can really only upgrade one component, and I think I have identified this component as the graphics card. I will post my full stats shortly but I do believe that this is the primary bottleneck in an otherwise quite capable system.

Manufacturer : Hewlett-Packard Company
Mainboard : Hewleet-Packard Asterope
Chipset : ATI RS400/RC400/RC410
Processor : Intel Pentium 4 524 @ 3066 MHz
Physical Memory : 2048 MB (2 x 1024 DDR2-SDRAM )
Video Card : ATI Technologies Inc Radeon XPRESS 200 SERIES (256MB Memory, Internal DAC at 400Mhz)
Hard Disk : ST3250823AS (250 GB)
DVD-Rom Drive : TSSTcorp CD/DVDW TS-H552D
Monitor Type : Hewlett Packard HP vs17 - 17 inches
Network Card : Realtek Semiconductor RT8139 (A/B/C/810x/813x/C+) Fast Ethernet Adapter
Operating System : Microsoft Windows XP Home Edition 5.01.2600 Service Pack 2
DirectX : Version 9.0c (October 2008)

I am correct in my reasoning that the thing I need to upgrade the most is the graphics card?