Hello, I would like some advice on the purchase of new computer hardware.

Here is my computer specs:

Microsoft Windows 7 Home Premium 64-bits Service Pack 1
Intel Core2 Duo CPU E8500 @ 3.16Ghz
4GB Ram Kingston
ATI Radeon Saphire HD 4890 1GB VRAM
No Sound Card (Onboard)
ASUS P5QL Pro
Hard Drive SATA 500GB
Full HD ASUS 1920x1080 (DVI, HDMI, VGA)

I can currently spend 260 dollars but I do not know what to buy first. You should know that I rarely play other games Oblivion (apart of course the next Elder Scrolls). I think from what I saw a card of 2GB of VRAM would be useful, and a sound card.
I intend to play with quite heavy mods such as FCOM, QTP3 and Better Cities. I prefer not to install RAEVWD because I think the graphics quality does not compensate for the loss of FPS.
Any opinions ?

Well, I'm no expert, but I'm going to have to say sound card. The onboard ones aren't that great afaik.

An SSD would probably improve your performance, but they are a bit pricey. Buying a small one with just the OS and Oblivion installed should work out well. The specs of that PC seem ok though. Are you really getting many performance issues?? The 2GB of VRAM would be needed for QTP3 (and I've heard may not even be enough in some cases), but you can get away with a 1GB card by choosing to install QTP3 Redimized (Reduced and Optimized). There's no real (noticeable) drop in quality, but large gains in performance.

meh, ive used an onboard sound card for 2 years now and never had any problem. i would defintely spend that money on a new video card. get the latest video card fermi or hd 6x series.

ok, thank you for your advices !

One more question, Is it worth to buy an other Radeon 4890 and put a crossfire ? Does Oblivion support crossfire ?

Oblivion doesn't make much use out of Crossfire, last time I checked, so probably isn't worth it.

Probably an SSD, for that money. Or a second hand graphics card upgrade. Or indeed, a very good gaming sound card. The best graphics you could hope for would be an HD6950, and I do not believe that (for Oblivion) the difference between that and your HD4890 would be worth the cost. Yes, 2GB is nice, but I am not sure it is a good idea spending ~$240 on that, just for Oblivion...

If I were you, I would save a little more, and upgrade your CPU + motherboard + RAM. That would set you up for Oblivion, sure, but also future games (e.g., TES V). At the moment, Sandy Bridge is your best bet, for gaming.