New vid card on old Computer-GT 240 or HD 4670 or ??

I have a 5 yr old computer. A gateway 825GM. My aftermarket video card (a X1600 pro) has died and I need a replacement. I was interested in upgrading it, and have done a bit research but had a few questions I wanted to ask the experts here.

Specs for my computer:
Gateway 825GM
Pentium 4 550 With Hyper-Threading 3.4GHz (yeah I know old)
4gb memory
PCI-E x16 (ver 1.0 I'm pretty sure) graphics slot
Upgraded power supply-600Watt I think

I was looking at 2 deals from NewEgg today. One is a PNY GeForce GT 240 512mb DDR5 for $45 AR. PNY LINK The other was Sapphire Radeon HD 4670 1gb DDR3 for $58 AR. SAPPHIRE LINK Both are PCI-E 2.0x16 cards (versus my old 1.0 x16 slot).

I mostly just play StarCraft 2 and Warcraft 3, so even though the processor is old, it can handle those 2 games. Everything else is internet, email, etc.

QUESTIONS
**I read where 2.0 x16 card will work on a 1.0 x16 slot but at the slower 1.0 throughput speed; is this true?
**Will the HDMI output on the card even work with the lower 1.0 slot throughput?
**Specs for the PNY GT 240 says it has built-in HDMI Audi Processor, so I don't need one of those S/PDIF cords (which, of course, my motherboard doesn't have) to pass audio out HDMI?
**Anyone know if the HD 4670 will process audio out HDMI without a PDIF cord input? I couldn't find it specifically called out
**TomsHardware shows the HD 4670 and GT 240 pretty close to each other on the hierarchy page, anyone have any experience with comparing one to the other?
**Which one would you recommend for my system: the GT-240 at $45 AR or the HD-4670 at $55 AR or JUST GET SOMETHING EVEN CHEAPER SINCE THE PERFORMANCE OF THESE 2 ARE TOO HAMPERED BY MY OLD COMPUTER?
**Should I just save the money and invest into a completely new computer

Thanks in advance for any insight & help.