I like the GIGABYTE GA-M57SLI-S4 AM2 NVIDIA nForce 570 SLI MCP ATX AMD Motherboard ($90)
. I have not used it in SLI, nor will I ever use SLI for anything; I got it because it seemed to be the best AM2 motherboard a couple months ago, regardless of SLI. It supports Phenom CPUs but you may need to flash the BIOS first (with your older 4200+), depending on the BIOS revision loaded, if you ever decide to upgrade to a Phenom.
If you go Intel,GIGABYTE GA-P35-DS3L LGA 775 Intel P35 ATX All Solid Capacitor Intel Motherboard ($90)
(no SLI).Intel Core 2 Duo 8400 - Dual Core, 3GHz, 65W ($190)
I do not generally suggest SLI, nor is it needed for a 1920x1080 screen, though gaming on a 51" 1080p TV is certainly the ideal application for it. If you must have it, you can use a $500 9800 GX2 (with 2 GPUs on a single card), which will fit into a single slot and thus work on any PCIe x16 motherboard.
If I were in that situation I would choose between the $300 9800 GTX, $500 9800 GX2, or (ideally) wait for the new generation of cards in late May / June from both ATI and nVidia. And if you wait, well, different motherboard chipsets will be out, too, like the P45 (ICH10) and AMD's SB750. But if you upgrade tomorrow...
If you get a 9800 GTX or lower ($180 8800GT or $130 9600GT both look quite nice), just buy a quality new motherboard for your AMD processor for around $100, keep your current CPU and RAM, and possibly upgrade it in a year once AMD comes out with newer, faster chips. With a 9600GT, by the way, you would still have the option of getting a second 9600GT in the AMD motherboard.
If you are going to spend $500 on a GX2, then you should really get a new CPU and 4GB of RAM. And in that case I would suggest an Intel motherboard and Intel CPU (either 8400 or 9300), since Intel is a bit faster in games right now (though their quad-cores are far too expensive for my taste). But unless you get a 9800GX2 or similarly powerful video setup, I don't think you will ever find yourself CPU-limited at 1920x1080 with antialiasing and other goodies enabled.
By the way - nVidia's nForce 570 for AMD is very good. But a lot of other nForce chipsets - especially Intel ones, like the 6xxi series - have problems like data corruption and I would personally not choose one over a P35. I'm not sure about the 750i.