The Seasonic would be fine with a GTS, a GTX would be really pushing it though, as you use a 19 inch screen you're unlikely to need more than a GTS and I doubt you will put a quad core to good use.
My suggestion would be to get a core 2 duo, 8800GTS and save around £100 for a new case, monitor, speakers (whatever you may want).
Asus P5K SE Intel P35 (Socket 775) PCI-Express DDR2 Motherboard £64.99
(£76.36)
Scythe Ninja Plus Rev.B CPU Cooler (Socket 478/754/939/940/AM2/LGA775) £24.99
(£29.36)
Intel Core 2 Duo E4400 "LGA775 Allendale" 2.00GHz (800FSB) - Retail £69.99
(£82.24)
OcUK GeForce 8800 GTS 320MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail £149.99
(£176.24)
GeIL 2GB (2x1GB) PC6400C4 800MHz Ultra Low Latency DDR2 Dual Channel Kit (GX22GB6400UDC) £49.99
(£58.74)
Sub Total : £359.95
Total = £422.94