Should i upgrade or buy new motherboard?

Sorry for not answering in the old thread.
But thx for the tips and tricks

But now when im thinking.
I am an *Gamer* played alot the last 6-7 years (Im 21)
And i wanna be able to play new games or atleast GTA IV!

So should i upgrade the things i can on this PC or by an new motherboard?

Which i use Now

Stolen School PC from 2004
http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?lang=en&cc=us&contentType=SupportManual&prodTypeId=12454&prodSeriesId=401819

Grapic card - Sparkle Geforce 8400GS 512 mb VRAM (Upgraded from the onboard 64 mb VRAM)
RAM - Upgraded from 512 mb DDR to 2 gig DDR
Process - Intel Pentium 4 2.8 Gigz (OC is locked)
HDD - 126 g Harddrive (Also upgraded)


Yeah i know that i should upgrade it but im running low on cash
Got somewhere around 200 - 280 euro every month after the bills are payed and meat shoped.
Currently i can play
Fallout 3 on medium settings without lag
same with COD4 , Diablo 3 etc

So what should i do?
Upgrade this shi.ty pc or buy an new motherboard?
In case of new motherboard , can someone
link me to an *Decent gaming full set* `?

Thx