I doubt it's the wifi that's the problem. Your 54g will run at 54 Mbps which is over 200 times faster than your 256 kbps internet connection.
When you say speed test are you testing speed to the internet using something like the ADSL Guide website Speed Tester?
If you want to test the bandwidth of your LAN, i.e. your wireless connection speed there's no point using an internet connection tester as your intranet speeds will always be several orders of magnitude faster.
To test your PC to PC connection you could transfer a very large file from one machine to the other, manually time it and then do the math. Alternatively you could use the Network Bandwidth Benchmark Test in SiSoft Sandra.
I think you'll find you are OK if you do these tests. You could also run the Windows Network Diagnostics in System Information.
I suspect your slow download speeds are due to the MTU and RWIN registry values in PC 2.
If you're not sure how to tweak these, post back (with the results of any of the above that you've run).
TOG