Here's a list of parts that I just threw together. Everything is on Newegg. The total is $626.97 shipped. You can save $40 if you get the Antec 300 case. I went with the three core processor, because it was only a few dollars more ($18) and it should make the computer last a bit longer. I also chose the ATI 4890 over the 4870, because they were almost the same price after the rebate. You will also want to consider an after market cooler if you plan on overclocking. I don't know how intensive Second Life is, but this should be able to handle most games very well on your TV.
Antec Nine Hundred Black Steel ATX Mid Tower Computer Case - Retail $99.95
ASUS M4A785T-M/CSM AM3 AMD 785G HDMI Micro ATX AMD Motherboard $84.99
HIS H489FN1GH Radeon HD 4890 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card $179.99 ($159 AR)
Antec EarthWatts EA650 650W Continuous Power ATX12V Ver.2.2 / EPS12V version 2.91 SLI Certified CrossFire Ready 80 PLUS Certified Active PFC $79.99
OCZ Gold 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1066 (PC3 8500) Dual Channel Kit Desktop Memory Model OCZ3G10664GK $82.99
AMD Athlon II X3 435 2.9GHz Socket AM3 95W Triple-Core Processor Model ADX435WFGIBOX $89.99