Should i upgrade my current rig?

[Conman]

Hi
I would to ask for opinion if i should upgrade my current rig.
AMD 64 4000+ with Syte Ninja (stock)
2 gb corsair 2.5-3-3-6-T1
A8N SLI premium
Winfast 7800 gt (stock)
HD 250 samsung SATA2
Tagan 580w
P180 antec

My system is alright, i am quite satisfy with its performace on games and my applications. I am a big fan of FPS and play CS source quite a lot. but i feel like i should upgrade it due the current market price.
I current concerns are

1. I understand that 7800 gt is no longer on market, it means my upgrade option is quite limited
2. If i want to sell my 7800 gt and go for 7900 gt or GTX my CPU and MB would not balance with this set up
3. 1800xt seems to be a good option however, i may have to change MB to satisfy the card
4.If i leave my current system like this will it be sensible to upgrade later without the chance to throw the system away.
If anyone please guide me with some advice and options within reasonble budget

I really can not make up my mind.

 

[Conman]

thanks for the above comments and advice

I forget to mention few things. Firstly, i am play my games on 1280x1024 res. I am surely do not want to spend more than 50 pounds on it unless it is very reasonable upgrade.

So far keeping the current system seems to be a good reason.
Anyway more comments would be great