Advice on new system

hi, i'm in a position to get some kind of upgrade over the next few weeks as an xmas present from the wife ;)
am looking for some input from u tech heads as to what to do...i wanna be able to play cod2, quake 4, fear, battlefield 2 with loads of detail and silky smooth gameplay

Firstly, my current system is :

p4 3.0ghz northwood 512kb L2 cache
Asus P4C800-E Deluxe
1GB Corsair TwinX (2xCMX256A-3200C2) + (2xCMX256A-3200C3)
gainward fx5900xt
120gb seagate barracuda 7200.7 sata
antec trueblue 480w
scan 3xs'2010' atx midi tower

this system has served me well over the past couple of years but is now struggling to play
the latest games.

i have roughly 600 quid to spend but am not sure which way to go

i am considering the following as the core of my new system = approx £480

AMD Athlon 64 3200 32/64Bit CPU S939 "Venice"
Asus A8N-SLi (Standard)
256MB XFX PCI-E GF7800GT

will i see a huge increase in performance with the above?

any ideas on case/psu i should opt for? am debating whether to get a Creative Labs SoundBlaster Audigy-4 7.1 OEM as i want to experience top sounds with my new medussa speedlink 5.1 surround sound headphones....will this card be a huge improvement over the onboard sound on the a8n?

any help would be much appreciated

thanks

on the sound front, the audigy four will be an improvement, although I seem to remember it being quite expensive, and the x-fi range are meant to be very good although also expensive

You should see a big increase with the 7800GT on the processor front if you are interested in overclocking the opteron 146 s939 would be a good bet as it is cheaper than the 3200venice same clock at stock speed and they tend to be awesome overclockers.

thanks for your input fredered..i'm basically decided on the 7800gt then.....the cpu side of things is confusing ;)

scan list the audigy 4 oem at £45 which sounds pretty good to me

scan list the opteron 146 s939 at £4.00 cheaper than the Athlon 64 3200 32/64Bit CPU S939 "Venice"..ok i see that the opteron has 1mb l2 cache and is very overclockable....have u got an opteron? how would the Asus A8N-SLi (Standard) match up with a opteron?

also my memory is 2.5 3 3 7 with 2 cl2 sticks and 2 cl3 sticks....giving the cl2.5...will this have an impact on overclocking an opteron?

on the case front i'm looking at the SilverStone "TEMJIN" J01S (Silver) combined with a
530W Tagan 2Force T530-U22 SLi U-Quiet 21db....any comments on these?

I have a Silverstone TJ06 case and I'm very impressed with it's build quality, so I wouldn't hesitate on buying another one.

hmm if i purchase a 7800gt card can i use my current power supply ie is it good enough and do i get a pci-e convertor with the card? - Antec TRueBlue 480w rated Max 38A +5V, 22A +12V, 30A +3.3V