Hi, got a decent gaming pc but theres a few parts i would like to buy in the near future and would like to know which one would most improve overall performance, or which would simply be most worthwhile, my current specs are:
Antec P180 case (3 generic 12cm fans)
Hiper 580W modular PSU
Asus A8N-Sli premium
Corsair XMS DDR400 PC3200 platinum 1Gb RAM (1Gb stick)
Geforce 7800Gt (XFX extreme edition)
Athlon 64 x2 4600 with standard heatsink/fan
Maxtor Diamond plus 10 300b S-ATA hard drive

The upgrades i've been considering are:
Another gig of Corsair XMS DDR400 PC3200 platinum RAM or
Creative Sound Blaster X-Fi Xtreme sound card or
1 Delta 120mm Focussed Flow Fan, 3 Vantec SF12025L Stealth 120mm fan's and an Akasa AK FC-03 Fan Controller Pro

All 3 of these upgrades cost about £100 each, which is the funding ive got, if anyone has any better ideas on how to spend the money please say as all ideas are welcome

The ram is the only upgrade that will affect your performance(and then only if you have need of 2gb, like some newer games do). Edit: I guess a sound card could make games run faster if you currently have integrated, so it could affect performance as well. The bad thing is if you use 4 memory dimms, it will force a 2t command rate on the ram(vs 1T), which makes the ram slower than if it was at 1T. If you need 2gb though, 2gb at 2t would be better than 1gb at 1t.

Another comment, I have one vantec stealth(92mm), and it's fairly quiet, but doesn't move very much air, but I guess that's the tradeoff of a quiet fan. Some people say to get a really loud fan that moves tons of air and put a controller on it to slow it down 'till it's quiet, that would be an option.

Well, I've got 1,5GB RAM myself, and commonly find myself using more than that and thus swapping to the pagefile...

But it depends on how you use your computer...

Keep taskmanager open when gaming etc and then see how much peak memory you have used, simple as that..

But you also say you only have one stick of RAM right now... Another stick will enable dual channel mode; somewhat improving the speed of your PC... Don't expect anything though, it does like 400 points in 3dmark for your setup...

one thing you might want to check if you go with another 1 gig stick is that the model of the ram is EXACTLY the same. i bought additional corsair xms ddr400 for my system thinking it was the same and it was actually slightly slower. i wound up running 2x512 instead of 3x512 so that i could run it at the full 400mhz in dual channel.

anyways, i have a similar system to yours (asus a8n-sli deluxe, 3700+ san diego, 2x512 corsair, 7800gt) and it tears through games such as quake 4, cod2, etc. 1gig gets it done for me, but since you have a dual core processor i would assume you go for multitasking at least part of the time.. so the extra ram could be more valuable for you.

If I was going for the 'ultra loud fans' i guess i'd have to choose 4 of the delta 120mm (there sumthing like 59dBA but spin at 4000rpm), I think if im just going for performance i'd have to choose the RAM but im just wondering how much a difference a sound card will make on the audio quality (currently i have my hi-fi speakers hooked up to the pc, but im hopefully either getting good headphones or a 5.1 surround sound setup and im using integrated sound). Also is it possible to hook up a fan that only has a molex connector to a fan control?

I would go for the new memory as i'm sure others will agree that it would make a significant improvement.
Also, some intergrated sound is pretty good (gaming wise) if you have issue with sound try updating the onboard sound drivers.

rv13uk; well, when you put it like that... I would myself go for the soundcard in that case... But that's just because I do listen to allot of music and when I am gaming really like the extra dimension a good quality soundcard gives over the regular AC97 audio "experience"

To put it into perspective; In my car I have audio equipment worth around 1000$... And my PC speaker setup cost me like 500$

Heh I dont have a car yet, but god knows that once i do, i'll have a real expensive audio system in my car =P i've even started it off with a 1000watt Mosfet car amp and a 400 watt car sub that i use for my Surround sound system in my room, hooked up to my computer, so yes, call me crazy, a 15 y/o boy has car audio stuff hooked up with a car battery and all in his room =P but hey sounds great.

BUT back to the subject, your Sound also has to do with your Equip uve got, like my integrated sound is good enough for the moment, of course i also have it hooked up to my Yamaha amp Via Fiber optic cable.

I have the Same Mobo uve got