Complete novice setting up VOIP

[Poppy]

I'm planning on trying out Blueface VOIP for a few months, then possibly getting rid of my landline permanently.

My broadband details are as follows:
ISP: NTL/UPC
Modem: NTL E08C007 (it has one ethernet port which is connected to my wireless router & one free USB port).
Wireless Router: Netgear 108Mbps WGT624 (it has one internet port connected to the modem & 4 free LAN ports)

Results with speedtest.net:
Download: 5,603 kbps
Upload: 284 kbps
43 ms ping

Suitablity for VOIP:
Jittter: 4.4ms
Packet loss: 0.0%
MOS score: 4.0

I have a Siemens Gigaset 4010 Classic (approx. 7 years old DECT phone).

I know that using a VOIP phone would probably give better quality than using an ATA (at least that's what I gather from posts on this forum). However, I'm reluctant to buy a new phone initially, as I'm just trying out VOIP.
1. Are the modem & wireless router that I have suitable for use with an ATA?
2. What is the best ATA that I can use?
3. How easy is it to install/use (I'm new to all this area) - is it just a case of connecting the phone to the ATA, then the ATA to the router/modem?
4. Can I switch back & forth between using the ATA & my landline connection if I want to?