Right, options, note IMHO, i'm not professing expert knowledge, just what ive been thinking for my gaming system, facing roughly the same issue as you, I have a 3200+ and a Radeon 9800Pro. A lot depends on your MB, the make of RAM etc.
1. SLI is a waste of good cash, the technology moves too quickly for the minimal advantage, and a single card you could buy tomorrow would probably kick the crap out of your SLI system today. If you were talking twin 7900's overclocked and all that entails and you were really an enthusiast about stretching every single FPS out then mebbe, but not with your budget. I dont understand SLI at all, I mean you get what, a 25% improvement over what you'd get with 1 card? crazy. But then im not an enthusiast.
2. So if we're talking bang for buck, youve got 2 options, AGP or PCI-E. The permutations are endless but i've been giving this some thought with my system. My two options are to expand my present AGP system or switch to PCI-E. Taking expand your present AGP:
GX-058-PC PowerColor ATI Radeon X850 XT 256MB DDR3 VIVO TV-Out/DVI (AGP) - Retail (GX-058-PC) £149.95 £149.95
MY-046-GL GeIL 2GB (2x1GB) PC3200 Value Dual Channel Kit CAS3 (GE2GB3200BDC) (MY-046-GL) £89.95 £89.95
SC-034-CL Creative Sound Blaster X-Fi Xtreme Music - Retail (SC-034-CL) £69.95 £69.95
Subtotal £309.85
Shipping (City Link Parcel Next Day (Delivered Mon-Fri)) £8.25
VAT £55.67
Total £373.77
I went for the X850XT over the 6800GS because I think it has better performance for the buck. The 7900GS im not a big fan of because of its limitations in the AGP slot. The RAM is overkill as well, you may be able to get it cheaper by just buying another 512 to match your existing RAM. You will probably also need a new PSU depending on what you have currently. I like the SB and with the extra cash you can buy a half decent speaker system or better you'd be thinking about buying a new system next year at the lastest.
3. Option 2 is PCI-E. I take it the 3400 is a S754 processor. So:
MB-113-AS Asus K8N4-E Deluxe nForce4 4X (Socket 754) PCI-Express Motherboard (MB-113-AS) £49.95 £49.95
MY-046-GL GeIL 2GB (2x1GB) PC3200 Value Dual Channel Kit CAS3 (GE2GB3200BDC) (MY-046-GL) £89.95 £89.95
GX-018-BG BFG GeForce 7900 GT OC 256MB GDDR3 TV-Out/Dual DVI (PCI-Express) - Retail (GX-018-BG) £209.95 £209.95
SC-000-HT HDA Digital X-Mystique 7.1 Gold PCI Dolby Soundcard (SC-000-HT) £46.95 £46.95
Subtotal £396.80
Shipping (City Link Parcel Next Day (Delivered Mon-Fri)) £8.95
VAT £71.01
Total £476.76
Hey presto a S754 PCI-E system which should keep you ticking over for a while. Again, the PSU issue, you will most definitely need a new one unless you have a decent one at the mo. The Mystique is meant to be a good card. I'd read through the other forums for specific advice on each component. Again at some point the Processor is going to limit your upgrade path, when that is I have no clue.
Cant say much about silencing and fans.
